V23N2 - JULY

July 2013
I N
T H I S
I S S U E
straighten out your power
source priorities 8
µModule DC/DC combines
Volume 23 Number 2
Minimize Standby Current in
Automotive DDR Supplies
David Gilbert
high efficiency switching
with low noise linear
regulation 18
reference clock distribution
for a 325MHz IF sampling
system with over 30MHz
bandwidth, 64dB SNR and
80dB SFDR 26
near noiseless ADC drivers
for imaging 31
When you turn on a laptop or a smart phone, you expect
to wait for it to boot up, but you are less patient when you
turn on your car. With a car, consumers expect immediate
access to computer electronics, including navigation and
infotainment systems, and automobile manufacturers strive to
meet this desire with design strategies that shorten start-up
time. One such strategy is to keep dynamic memory (RAM)
active at all times, even during the ignition-off state.
The DDR3 memory used in automobiles operates on a 1.5V rail with peak load currents over
2A—preferably produced by a high efficiency DC/DC converter to minimize heat dissipation. In
these applications, light load efficiency is just as important to preserve battery life when the automobile is not
running. DDR memory can consume 1m A–10m A from the
1.5V rail in standby, but drawing 10m A from the battery
is unacceptable when the car is parked for long periods.
This constraint rules out the use of a linear regulator, where
input and output current are equal. On the other hand, a
switching step-down (buck) regulator draws less input current than load current in proportion to the step-down ratio:
IIN =
1 VOUT • IOUT
•
VIN
η
where h is the efficiency factor (0 to 1).
Figure 1 shows that the LT®8610AB synchronous stepdown regulator achieves ~83% efficiency at a 1m A load.
For a battery voltage of 12V and load current of 1m A at
1.5V, calculated input current is only 151µ A.
The LT8610 keeps automotive electronics running.
Caption
w w w. li n ea r.com
(continued on page 4)
Linear in the News
In this issue...
COVER STORY
Minimize Standby Current in Automotive DDR Supplies
David Gilbert
1
DESIGN FEATURES
Prioritize Power Sources in Any Order, Regardless
of Relative Voltage: No µP Required
Sam Tran
8
3mm × 3mm QFN IC Directly Monitors 0V to
80V Supplies: Features I2C Interface, Peak
Value Tracking and Runs from Any Supply
CY Lai
13
High Efficiency Switching Power Conversion Combined
with Low Noise Linear Regulation in µModule Package
David Ng
18
µModule® Regulator Powers and Protects
Low Voltage µProcessors, ASICs and FPGAs
from Intermediate Bus Voltage Surges
Willie Chan and Jason Sekanina
20
DESIGN IDEAS
What’s New with LTspice IV?
Gabino Alonso
24
Reference Clock Distribution for a 325MHz
IF Sampling System with over 30MHz
Bandwidth, 64dB SNR and 80dB SFDR
Michel Azarian
26
Near Noiseless ADC Drivers for Imaging
Derek Redmayne
Published by the Newnes imprint of Elsevier Science & Technology Books
in January, Linear’s Analog Circuit Design, Volume 2 is gaining readers. The
book, edited by industry gurus Bob Dobkin and the late Jim Williams, is the
second volume of the series, covering a broad range of analog circuit design
techniques. Analog Circuit Design, Volume 2, Immersion in the Black Art of
Analog Design has received positive reviews in various electronics publications.
EDN is currently running excerpts of the book’s chapters on their website.
EDN Analog and Power editor Steve Taranovich commented, “This book is a great
companion volume to Volume I with informative application notes and a full
complement of reference designs. The chapters are not just every day application
notes and reference designs, but give insights into problem-solving, design decision-making, the thought process that goes along with a robust, successful design.
That’s why I love this book. The authors, Bob Dobkin and the late Jim Williams
(along with the research and writings of Carl Nelson and Bob Widlar), have a rich
history and depth of experience that they share with the readers. This book is a
keeper that needs to be on every designer’s bookshelf, right next to Volume I.”
A recent article in UK’s Electronics Weekly commented, “It is a book that
delves into the deepest subtleties of linear analog electronics and, unless you
have no imagination, and just want to copy a reference design, or you are
already a grade-A analog guru, you will learn something from every page.”
In April, at the Design West Show, held in San Jose, California, Bob Dobkin discussed the art of analog design, took questions from engineers and signed copies
of both volumes of the Analog Circuit Design book series. On display, along with
31
Low Power, DC Accurate Drivers for 18-Bit ADCs
Guy Hoover
36
back page circuits
40
2 | July 2013 : LT Journal of Analog Innovation
ANALOG CIRCUIT DESIGN BOOK MAKES WAVES
Bob Dobkin signs Analog
Circuit Design, Volume 2
at Design West
Linear in the news
the book, were photos of Jim Williams’
innovative electronic sculpture art, which
Jim painstakingly assembled from found
electronics that he personally scoured
from his frequent trips to electronics flea
markets. In addition to being a work of
art, each of Jim’s sculptures is a working
electronic instrument, providing accurate readings of temperature, barometric
pressure and other natural phenomena.
At the book signing, Bob Dobkin discussed the challenges and satisfactions
of analog circuit design. Following is an
excerpt from Bob Dobkin’s Foreword to
Analog Circuit Design, Volume 2:
“Analog design is challenging. There are
many ways to get from input to output,
and the circuitry in the middle can lead
to divergent results. Analog design is like
learning a language. When you first learn
a language, you begin with a vocabulary
book and then analyze writings in that
language by looking up words one by
one as you encounter them. Likewise, in
analog design you learn the basics of the
circuit, as well as the function of different devices. You can write node equations
and determine what the circuit is doing by
studying each of the individual circuits.
“With analog circuit design, you end up
using the basic circuit configurations you
have learned—differential amplifiers,
transistors, FETs, resistors, and previously
studied circuits—to achieve the final
circuit. As with a new language, it takes
many years to learn to write poetry, and
the same is true of analog circuit design.
“Today’s system manuals rarely have
any circuit design. They contain block
diagrams and hookup schematics with
thousands of leads to different blocks.
So where do people go for analog circuit
design? If you’re just starting, or if you
encounter a problem that’s outside your
experience, appropriate references are
difficult to find. Hopefully, these books
Linear’s Analog Circuit Design book
series, published by Elsevier. Volume 1
was Elsevier’s best selling engineering
title last year and Volume 2, published in
January, is off to a strong start.
provide some answers, as well as circuit
design and test and lab techniques for
design and duplication of the circuits.
“Today, analog design is in greater
demand than ever before. Analog design
is now a combination of transistors
and ICs that provide high functionality in analog signal processing. This
volume focuses on fundamental aspects
of circuit design, layout, and testing. It
is our hope that the talented writers of
these application notes shed some light
on the ‘black art’ of analog design.”
For more information on the book,
including author videos and ordering information, visit www.linear.
com/designtools/acd_book.php
LINEAR PRODUCTS
RECEIVE AWARDS
Several Linear products have recently been
honored with significant industry awards:
Synchronous rectification delivers efficiency as high as 96% while Burst Mode®
operation technology keeps quiescent
current under 2.5µ A in no load standby
conditions. The device’s ultralow quiescent
current makes it well suited for applications such as automotive or industrial
systems, which demand always-on
operation and optimum battery life.
EE Times China ACE Award for Best Data Conversion
Product, LTC2389-18—The LTC2389-18 is
the fastest 18-bit no latency SAR ADC on
the market with unrivaled 99.8dB SNR at
2.5Msps. The device’s high precision makes
it ideal for applications in medical imaging, high speed data acquisition, scientific
instrumentation, industrial process control,
automatic test equipment, radar, sonar
and semiconductor production equipment.
CONFERENCES & EVENTS
Techno Frontier 2013, Power System Show,
Tokyo Big Sight, Tokyo, Japan, July 17-19, Booth
EE Times & EDN ACE Award for Best Power Device,
1F301—Linear will showcase high perfor-
LTC6804 Battery Stack Monitor—The LTC®6804
mance analog solutions, with products
displayed in power system management, battery management systems and
wireless sensor networks. More at
www.jma.or.jp/tf/en/index.html
is a high voltage battery monitor for
hybrid electric and electric vehicles,
and other high voltage, stacked-battery
systems. An LTC6804 can measure up
to 12 series-connected battery cells at
voltages up to 4.2V with 16-bit resolution and better than 0.04% accuracy.
EE Times China ACE Award for Best Power
Management Product, LT8610/LT8611—The
LT8610 is a 2.5A, 42V input capable synchronous step-down switching regulator.
The Battery Show 2013, Suburban Collection
Showcase, Novi, Michigan, September 17-19,
Booth E1011—Presenting Linear’s battery
management system products. Sam Nork
will present “Active balancing solutions for series-connected battery packs”
at 12:05 pm, Thursday, September 19.
More at www.thebatteryshow.com n
July 2013 : LT Journal of Analog Innovation | 3
The LT8610A and LT8610AB have a low component count, low
minimum input voltage, low quiescent current and high efficiency across
a wide load range. These features make them the preferred solution for
providing standby power to DDR memory in automotive applications.
(LT8610A/AB continued from page 1)
100
DIRECT DC/DC CONVERSION
FROM CAR BATTERY TO
1.5V DDR MEMORY
95
EFFICIENCY (%)
90
LT8610A and LT8610AB are monolithic,
synchronous step-down regulators
designed specifically for automotive
systems. They deliver 3.5A while consuming only 2.5µ A quiescent current. Building
a circuit around them is easy. No additional semiconductors are required, they
work with inexpensive ceramic capacitors, and the MSOP package has leads that
are easy to solder and inspect. With a
typical minimum on-time of 30ns (45ns
guaranteed maximum), one can design
compact, high switching-frequency
buck regulators with large step-down
ratios. Figure 2 shows an application
that delivers 3.5A at 1.5V. The operating
frequency is 475kHz to optimize efficiency
and remain below the AM radio band.
Figure 2. This LT8610A or
LT8610AB step-down converter
circuit accepts automotive
battery and generates 1.5V at
3.5A. Low quiescent current and
synchronous rectification result in
high efficiency across the entire
load range.
4 | July 2013 : LT Journal of Analog Innovation
4.7µF
80
75
70
65
60
55
50
1
0.1
1k
10k
Figure 1. LT8610AB efficiency versus load
input is worst-case 3.4V, the maximum
duty cycle is above 99% and the dropout voltage is typically 200mV at 1A, all
of which keep the output in regulation
through cold-crank. The typical minimum
input voltage is plotted in Figure 3.
The difference between the LT8610A and
LT8610AB is that the LT8610AB features
higher efficiency at light loads. This is
achieved by using an increased Burst
Mode current limit, allowing more
energy to be delivered during each
switch cycle and lowering the switching frequency for a given load. Because
a fixed amount of energy is required to
SAVE THE BATTERY WITH LOW
RIPPLE Burst Mode OPERATION AND
MINIMAL QUIESCENT CURRENT
VIN
BST
EN/UV
LT8610A/
PG
SW
LT8610AB
BIAS
SYNC
FB
TR/SS
INTVCC
RT
93.1k
fSW = 475kHz
GND
0.1µF
3.3µH
549k
4.7pF
1M
3.6
3.4
3.2
INPUT VOLTAGE (V)
The LT8610A and LT8610AB are designed
to minimize output voltage ripple over
the entire load range. At light loads, they
10nF
1µF
10
100
ILOAD (mA)
VIN = 12V
VOUT = 1.5V
RT = 93.1k (475kHz)
L = COILCRAFT XAL6030-332ME 3.3µH
Both parts feature excellent fault tolerance
to automotive environments. A maximum input of 42V handles load dumps.
A robust switch design and high speed
current comparator protect the device
during output shorts. The minimum
VIN
12V
85
maintain efficiency by reducing their operating frequency and entering Burst Mode®
operation. Fast transient response is maintained even at very low loads. This feature
combined with the very low quiescent
current of 2.5µ A means that, even at a few
µ A of load, the LT8610A and LT8610AB are
more efficient than a linear regulator
with zero quiescent current. For systems
where low frequency operation must be
avoided, Burst Mode operation can be
turned off by applying either a logic high
signal or clock signal to the SYNC pin.
3.0
2.8
2.6
2.4
VOUT
1.5V
47µF 3.5A
×3
2.2
2.0
–55 –25
95
65
35
TEMPERATURE (°C)
5
125
155
Figure 3. Keeping the memory alive in cold-crank
or start-stop events. The LT8610A and LT8610AB
operate down to a typical minimum input voltage of
2.9V at 25°C, with 3.4V maximum guaranteed over
temperature.
design features
The LT8610A and LT8610AB are designed to minimize output voltage
ripple over the entire load range. At light loads, they maintain efficiency by
reducing their operating frequency and entering Burst Mode operation.
Fast transient response is maintained even at very low loads.
EFFICIENCY (%)
90
OUTPUT VOLTAGE RIPPLE (mVP–P)
95
LT8610AB
85
80
LT8610A
75
70
65
60
55
50
1
0.1
10
100
ILOAD (mA)
1k
10k
70
70
60
60
50
40
30
20
10
0
VIN = 12V
VOUT = 1.5V
RT = 93.1k (475kHz)
L = COILCRAFT XAL6030-332ME 3.3µH
switch the MOSFET on and off, a lower
switching frequency reduces gatecharge losses and increases efficiency.
Figure 4 shows the efficiency difference
between the LT8610A and LT8610AB.
For loads between 1m A and 100m A, the
LT8610AB improves efficiency by more
than 10% compared to the LT8610A.
The trade-off to the increased Burst
Mode current limit is that more energy
is delivered in each switch cycle, so
VIN
4.7µF
BST
EN/UV
LT8610A/
PG
SW
LT8610AB
BIAS
SYNC
10nF
FB
TR/SS
1µF
INTVCC
RT
18.2k
fSW = 2MHz
GND
LT8610A
2
1
3
NUMBER OF 1210 47µF OUTPUT CAPACITORS
VIN = 12V
VOUT = 1.5V
RT = 93.1k (475kHz)
L = COILCRAFT XAL6030-332ME 3.3µH
Figure 4. An increased Burst Mode current limit on
the LT8610AB results in substantial efficiency gains
at light load compared to the LT8610A. VIN
12V
LT8610AB
0.1µF
1µH
549k
(a)
50
40
30
20
LT8610AB
10
0
LT8610A
2
1
3
NUMBER OF 1210 47µF OUTPUT CAPACITORS
VIN = 12V
VOUT = 1.5V
RT = 18.2k (2MHz)
L = COILCRAFT XAL4020-102ME 1µH
(b)
Figure 5. Output voltage ripple versus number of 1210 size 47µF output capacitors for two inductor values, at
10mA load. (a) Ripple for the 475kHz application in Figure 2. (b) Ripple for the 2MHz application in Figure 6.
more output capacitance is required
to keep output voltage ripple low.
Figure 5 compares the output ripple for
the LT8610A and LT8610AB as a function of the output capacitance for
two inductor values, at 10m A load.
one. If high efficiency at light loads is
paramount, then the inductor value
can be increased beyond the starting
value recommended in the data sheet.
In addition to the current limit, the
inductor choice affects the efficiency
and switching frequency in Burst Mode
operation. This is because for a fixed
current limit, a larger inductor value
can store more energy than a smaller
For most automotive systems, 9V to
16V is the typical input voltage, so application circuits are usually optimized
for this range. The 475kHz application
in Figure 2 operates at the designed
frequency over the entire input range
of 3.5V to 42V. However, if we restrict
the normal operating voltage to
16V (42V transient), the operating frequency can be increased and the value
and size of the inductor reduced. With
a worst-case minimum on-time of 45ns,
the LT8610A and LT8610AB can be programmed to 2MHz as shown in Figure 6.
VOUT
1.5V
47µF 3.5A
×3
4.7pF
1M
OUTPUT VOLTAGE RIPPLE (mVP–P)
100
Figure 6. Similar 12V to 1.5V application as in
Figure 2, but the operating frequency of the LT8610A
and LT8610AB is increased to 2MHz for reduced
inductor value and size.
GO FASTER FOR A SMALLER
SOLUTION
July 2013 : LT Journal of Analog Innovation | 5
100
100
95
95
90
90
COILCRAFT
XAL5030-222ME 2.2µH
85
EFFICIENCY (%)
EFFICIENCY (%)
An important feature is that this internal regulator can draw current from either the VIN pin
or the BIAS pin. If a voltage of 3.1V or higher is tied to the BIAS pin, gate drive current
is drawn from BIAS. If the BIAS voltage is lower than VIN, the internal linear regulator will
run more efficiently using the lower voltage supply, thus increasing overall efficiency.
80
75
COILCRAFT
XAL4020-102ME 1µH
70
65
80
75
65
55
55
1
10
100
ILOAD (mA)
1k
50
10k
COILCRAFT
XAL4020-102ME 1µH
70
60
0.1
The LT8610A and LT8610AB use two internal nMOSFETs specifically optimized for
automotive applications. In particular, the
gate drive circuitry requires less than 3V to
fully enhance the FETs. To generate the
gate drive supply, the LT8610A/AB includes
an internal linear voltage regulator, the
output of which is the INTVCC pin (do not
load INTVCC with external circuitry).
85
60
50
BIAS PIN OPTIMIZES EFFICIENCY
LT8610AB
VIN = 12V
VOUT = 1.5V
RT = 18.2k (2MHz)
COILCRAFT
XAL5030-222ME 2.2µH
0.1
1
10
100
ILOAD (mA)
1k
10k
An important feature is that this internal
regulator can draw current from either
the VIN pin or the BIAS pin. If the BIAS pin
is left open, then gate drive current is
drawn from VIN . However, if a voltage
of 3.1V or higher is tied to the BIAS pin,
gate drive current is drawn from BIAS.
If the BIAS voltage is lower than VIN, the
internal linear regulator will run more
efficiently using the lower voltage supply, thus increasing overall efficiency.
LT8610A
VIN = 12V
VOUT = 1.5V
RT = 18.2k (2MHz)
Figure 7. LT8610A and LT8610AB efficiency versus load at 2MHz with two inductor values
100
100
95
95
90
BIAS = 3.3V
GATE DRIVE DRAWN FROM
85% EFFICIENT SUPPLY
85
80
75
EFFICIENCY (%)
EFFICIENCY (%)
90
BIAS = 0V
GATE DRIVE DRAWN FROM VIN
70
65
85
80
70
65
60
60
55
55
50
0.1
1
10
100
ILOAD (mA)
1k
10k
LT8610AB
VIN = 12V
VOUT = 1.5V
RT = 18.2k (2MHz)
L = COILCRAFT XAL4020-102ME 1µH
BIAS = 3.3V
GATE DRIVE DRAWN FROM
85% EFFICIENT SUPPLY
75
50
BIAS = 0V
GATE DRIVE DRAWN FROM VIN
0.1
1
10
100
ILOAD (mA)
1k
10k
LT8610A
VIN = 12V
VOUT = 1.5V
RT = 18.2k (2MHz)
L = COILCRAFT XAL4020-102ME 1µH
Figure 8. Efficiency can be increased if the BIAS pin is tied to an external 3.3V supply. (External supply
efficiency of 85% is assumed and factored into overall efficiency shown here.)
Note that when the input voltage goes
above 16V, the output remains in regulation although the switching frequency
decreases to maintain safe operation. The
2MHz solution is identical to the circuit
in Figure 2, except for the RT resistor
6 | July 2013 : LT Journal of Analog Innovation
changed to 18.2kΩ and the inductor
value and size reduced in order to save
space. Figure 7 shows efficiency versus load for two inductor choices.
The efficiency data in Figures 1, 4 and 7
was recorded with the BIAS pin open. After
all, if the 1.5V output is the only rail alive,
then there is likely no good place to tie
the BIAS pin. However, if there is a 3.3V or
5V supply, tie it to the BIAS pin, even if
the supply is not available in standby or
ignition-off conditions. Figure 8 shows the
efficiency with and without a 3.3V supply
connected to BIAS. In calculating the total
efficiency, we have included the power
drawn from the 3.3V rail and assumed
that it was generated with 85% efficiency.
Note that the benefit to externally powering BIAS is greater at higher operating
frequencies because the gate drive current is higher. The LT8610A also benefits
design features
An important consideration for automotive applications is the behavior of the
power supply during cold crank and idle-stop transients, when the voltage from
the 12V battery may drop below 4V. The LT8610AB operates up to 99% duty
cycle, providing output regulation at the lowest possible input voltage.
more from external bias compared to
the LT8610AB—the AB’s increased Burst
Mode current limit results in a lower
operating frequency for a given load.
and reliable output voltage as a function of input voltage. Figure 10(b) shows
the output voltage as the input supply is
ramped from zero to 10V and back to zero.
NOT JUST FOR MEMORY
CONCLUSION
The LT8610AB is an excellent regulator
for other automotive supplies, including 3.3V and 5V supplies, with efficiency
above 90%, as shown in Figure 9.
The LT8610A and LT8610AB have a low
component count, low minimum input
voltage, low quiescent current and high
efficiency across a wide load range.
These features make them the preferred solutions for providing standby
PARAMETER
LT8610
LT8610A
LT8610AB
Max Load Current (A)
2.5
3.5
3.5
Minimum On-Time
(ns) (Typ)
50
30
30
V IN = 12V, V OUT = 1.5V, I LOAD = 10mA,
f SW=475kHz, L = 3.3µH
73.9
73.9
85.5
Output Voltage Ripple
(mV P-P)
I LOAD = 10mA, C OUT = 47µF, L = 3.3µH
8.4
8.4
52.5
800
90
700
80
600
60
50
40
30
fSW = 700kHz
VIN = 12V
L = 4.7µH
20
0.001
0.1
VOUT = 3.3V
VOUT = 5V
1
10
100
LOAD CURRENT (mA)
1000
Figure 9. Efficiency for 3.3V and 5V outputs is above
90%, reducing total power dissipation and keeping
temperature under control.
CONDITIONS
Efficiency (%)
100
70
Visit www.linear.com/LT8610 for
data sheets, demo boards and other
applications information. n
Table 1. Comparison of features for the LT8610 family of monolithic, synchronous buck converters
DROPOUT VOLTAGE (V)
EFFICIENCY (%)
An important consideration for automotive applications is the behavior of the
power supply during cold crank and
idle-stop transients, when the voltage from
the 12V battery may drop below 4V. The
LT8610AB operates up to 99% duty cycle,
providing output regulation at the lowest possible input voltage. Figure 10(a)
shows the dropout voltage. This is the
difference between VIN and VOUT as the
input voltage decreases towards the
intended output regulation voltage. The
LT8610AB also has excellent start-up and
dropout behavior, resulting in predictable
power to DDR memory in automotive
applications. Table 1 summarizes the
performance of the LT8610 family.
VIN
2V/DIV
VIN
500
400
VOUT
2V/DIV
300
VOUT
200
100
0
0
0.5
1.5
2
2.5
1
LOAD CURRENT (A)
(a)
3
3.5
100ms/DIV
2.5Ω LOAD
(2A IN REGULATION)
(b)
Figure 10. The LT8610AB operates to 99% duty cycle, providing smooth start-up and low dropout voltage.
July 2013 : LT Journal of Analog Innovation | 7
Prioritize Power Sources in Any Order, Regardless of Relative
Voltage: No µP Required
Sam Tran
Does your application have multiple input power sources? Are one or more secondary
source voltages equal to or higher than the main source? How do you ensure the
main source powers the output when a higher voltage, secondary source is present?
How do you prevent sources from cross conducting, or backfeeding during input
source switchover? Do you need to prevent current sharing between similar voltage
sources? Are you worried about users plugging in sources backwards or plugging in
overvoltage sources to the system? The LTC4417 prioritized PowerPath™ controller,
with its wide 2.5V to 36V operating voltage range, solves all these issues by controlling
the connection of the input sources based on user-defined priority and validity while
protecting the system from overvoltage and reverse voltage insertions up to ±42V.
PRIORITIZING THREE
INPUT SOURCES
Figure 1 shows a triple-input prioritizer.
Here, the 12V wall adapter is given top
priority, the 14.8V Li-ion battery stack is
prioritized second, and 12V sealed lead
acid (SLA) battery has the lowest priority.
Priority is simply set by connecting the
sources to the LTC4417 in pin order: V1 is
the highest priority; V3 the lowest priority.
The LTC4417 connects a higher priority input source to the output as long
as the input source voltage remains
valid—i.e., within its resistive-dividerdefined overvoltage (OV) and undervoltage (UV) window. As long as the higher
priority source remains valid, lower
priority inputs remain disconnected,
regardless of their relative voltages.
Accurate (±1.5%) comparators continuously monitor each input’s OV and UV pins
to ensure an input source is stable for at
least 256ms before validating and allowing
a connection to the output. Input sources
are quickly disconnected if an OV or
FDD4685
V1
12V WALL
ADAPTER
V2
14.8V Li-ION
MAIN/SWAPPABLE
FDD4685
FDD4685
+
+
FDD4685
CL
120µF
CS
6.8nF
RS
1k
DS
BAT54
FDD4685
V3 +
12V SLA
BACKUP
0.1µF
0.1µF
FDD4685
0.1µF
0.1µF
V1
VS1 G1
VS2 G2
806k
VS3 G3
VOUT
UV1
1M
39.2k
OV1
60.4k
0.1µF
UV2
V1 INVALID
V2 INVALID
VALID3
V3 INVALID
LTC4417
31.6k
OV2
0.1µF
68.1k
V3
EN
SHDN
HYS
CAS
698k
UV3
16.9k
OV3
1M
VALID2
V2
1.05M
1M
VALID1
GND
49.9k
Figure 1. Triple input LTC4417 prioritized input source selection application
8 | July 2013 : LT Journal of Analog Innovation
2A
OUTPUT
design features
The high voltage (2.5V to 36V), triple input LTC4417 prioritized PowerPath controller
is easy to use, robust and complete. Automatically prioritized supply current
sourcing extends the life of lower priority input sources while controlled switching
protects input sources from cross and reverse conduction during switchover.
12V WALL ADAPTER VALIDATES
V2
2V/DIV
V1, VOUT
2V/DIV
14.8V Li-ION BATTERY
12V WALL ADAPTER
VOUT
UNDERVOLTAGE FAULT
IWALL, IBAT
5A/DIV
14.8V Li-ION BATTERY
V2, VOUT
2V/DIV
VOUT
V1
2V/DIV
IBAT
1A/DIV
12V WALL ADAPTER
Li-ION BATTERY CURRENT
WALL ADAPTER CURRENT
Li-ION BATTERY CURRENT
IWALL
1A/DIV
WALL ADAPTER CURRENT
100µs/DIV
IL = 2A
CL = 120µF
100µs/DIV
IL = 2A
CL = 120µF
Figure 2. Switchover from a lower voltage input to a
higher voltage input
Figure 3. Switchover from a higher voltage to a lower
voltage input
UV condition is sensed. An internal 8µs OV,
UV filter time helps prevent false tripping.
range. An integrated 6.2V gate to source
clamp prevents gate-to-source oxide
overvoltage stress while allowing sufficient overdrive to enhance common
logic level rated P-channel MOSFETs.
Switching over to another valid input
source can only occur when an OV or
UV fault is detected or a higher priority
source becomes valid. Referring again
to Figure 1, this allows the lower voltage, higher priority 12V wall adapter to
remain connected to the output provided
it is valid. If another source is powering the output, the 12V wall adapter
is reconnected to the output as soon
as the wall adapter becomes valid.
The LTC4417 drives external back-to-back
P-channel MOSFETs as switches to connect
and disconnect input supplies to and from
the output. Strong gate drivers ensure
the back-to-back P-channel MOSFETs are
firmly held off during input source insertion and provide enough strength to drive
large, low RDS(ON), P-channel MOSFETs for
reduced steady state power dissipation
and increased output operating voltage
An important feature of the LTC4417 is the
break-before-make circuit that protects
input sources from cross-conduction
during switchover. Gate-to-source (VGS)
comparators sense that the external
MOSFETs of the disconnecting input source
are off before another input source is
allowed to connect to the common output.
To prevent reverse conduction from the
output to an input source during connection, reverse voltage (REV) comparators
delay the connection if a higher output
voltage is detected. Connection is delayed
until the output voltage drops below
the connecting input source voltage.
Figure 2 captures the event when the
LTC4417 disconnects the 12V wall adapter
from the output due to a UV fault. Once
the VGS comparator confirms that the
disconnecting 12V wall adapter’s backto-back P-channel MOSFETs are off, the
next highest priority valid input source,
the 14.8V Li-ion battery, is immediately
connected to the output. The two input
source current waveforms show that
no cross or reverse conduction occurs
between the input sources during switch­
over, thanks to the VGS comparator.
A resistor and capacitor, RS and CS in
Figure 1, serve to limit the 14.8V Li-ion
battery inrush current to a peak of
14A when it connects to the output. High
inrush currents can cause input source
UV faults, exceed the external MOSFET’s
maximum pulsed drain current (IDM), or
potentially damage connectors. The addition of RS and CS increases the switchover time, resulting in an output voltage
droop of 400mV. Note that larger RS and
CS values result in lower inrush currents
at the expense of additional output voltage droop. Keep this trade-off in mind
when selecting RS and CS . The Schottky
diode, DS, preserves the strong turn-off.
Figure 3 shows the LTC4417 disconnecting the lower priority valid 14.8V Li-ion
battery stack to allow the newly validated
higher priority 12V wall adapter to connect to the output. The REV comparator
senses the initial 14.8V output voltage
and prevents the 12V wall adapter from
immediately connecting to the output.
The REV comparator delays the connection until the output discharges below
the 12V wall adapter voltage to ensure no
reverse current occurs, as shown by the
two input source current waveforms.
July 2013 : LT Journal of Analog Innovation | 9
The LTC4417 drives external back-to-back P-channel MOSFETs as switches to connect
and disconnect input supplies to and from the output. Strong gate drivers ensure the
back-to-back P-channel MOSFETs are firmly held off during input source insertion and
provide enough strength to drive large, low RDS(ON), P-channel MOSFETs for reduced
steady state power dissipation and increased output operating voltage range.
V1
2V/DIV
VOUT
2V/DIV
IIN
0.2A/DIV
PRIORITIZED, LOW I CC MINIMIZES
POWER DRAW
12V WALL ADAPTER
The LTC4417 draws only 28µ A of total
operating current, and it draws as much of
this as possible from the highest priority
valid supply. During normal operation,
more than half the supply current is drawn
from the output when VOUT is above 2.5V.
When VOUT is less than 2.5V, operating
current is drawn from the highest priority valid input supply, with any remaining
supply current sourced from the highest voltage input source. The LTC4417
consumes almost no current from lower
priority input sources when their voltages are lower than the output voltage.
VOUT
IIN
1ms/DIV
IL = 0A
CL = 120µF
Figure 4. Output soft-start
Inrush current is also limited from the
12V wall adapter because it is quickly
switched in when the output voltage is
11.88V. As its current waveform shows,
the wall adapter provides the 2A load
current plus the small additional current necessary to charge VOUT.
When SHDN is forced low, the part is
placed into a suspended mode where
the OV and UV comparators are powered down to conserve power and
all input sources are invalidated. In
this state, the supply current is drawn
from the highest voltage source.
Figure 5. 24V Application with reverse voltage protection
V1
24V REGULATED
SWITCHING
TABLETOP SUPPLY
SMBJ26CA
FDD4685
High inrush currents can occur when
a higher voltage input source quickly
connects to a lower voltage output
bulk capacitor. When the output voltage is less than 0.7V, the LTC4417 soft
starts VOUT to minimize inrush current.
Figure 4 shows the input current and output voltage waveforms when the LTC4417
soft-starts from the 12V wall adapter
to an initially discharged 120µ F output
bulk capacitor. As the figure shows, the
peak input current is limited to 500m A.
After the output has been connected to its
first supply, systems with similar voltages have minimal inrush current when
changing channels due to the similar
input and output voltages during input
source switchover. This allows systems
with similar input source voltages to
omit the RS, CS, and DS inrush current
limiting circuitry shown in Figure 1.
Figure 6. Over and reverse voltage blocking
FDD4685
+
FDD4685
V2
14.8V Li-ION
MAIN BATTERY
OUTPUT SOFT-START
2A
OUTPUT
CL
120µF
V3, VOUT
10V/DIV
V3 = VOUT = 11.1V
27V
FDD4685
FDD4685
V3
11.1V Li-ION
BACKUP BATTERY
FDD4685
V1
10V/DIV
–27V
V1
VS1 G1
VS2 G2
V2
V3
LTC4417
VS3 G3
VOUT
100µs/DIV
IL = 2A
CL = 120µF
10 | July 2013 : LT Journal of Analog Innovation
design features
With input source pins, V1 to V3, designed to handle ±42V, the LTC4417
only requires that the external P-channel MOSFETs be chosen with a BVDSS
rating greater than any anticipated voltage excursions from input to output for
seamless over, under and reverse voltage input source insertion protection.
OVERVOLTAGE, UNDERVOLTAGE
AND REVERSE VOLTAGE
INSERTION PROTECTION
Applications where sources are physically plugged in and unplugged face the
possibility of improper or faulty source
insertions. Faulty wall adapter insertions
can expose the system to potentially
damaging overvoltage events while reverse
voltage insertion can occur from improperly inserted batteries. These miscues can
be compounded by the prevalence of
standardized connectors with differing
voltage specifications. With input source
pins, V1 to V3, designed to handle ±42V,
the LTC4417 only requires that the external P-channel MOSFETs be chosen with a
BVDSS rating greater than any anticipated
voltage excursions from input to output
for seamless over, under and reverse voltage input source insertion protection.
Figure 5 shows a complete input
fault insertion protected system. The
LTC4417 protects itself against input
voltage ranging from –42V to 42V. The
–40V BVDSS FDD4685 P-channel MOSFETs
are chosen to withstand the worst-case
voltage excursion. During insertion, a
256ms deglitch timer ensures the strong
gate drivers initially hold the external
MOSFETs off. Transient voltage suppressor
(TVS) diodes, highly recommended with
input voltages above 20V, ensure transient
voltage excursions do not exceed the
LTC4417’s absolute maximum voltage of
±42V. Figure 6 shows the LTC4417 blocking
a forced V1 overvoltage step of 27V and
subsequent –27V reverse voltage step
from the 11.1V Li-ion battery stack and
V1
12V SUPPLY
R3
806k
R2
41.2k
V2
7.4V Li-Ion
BATTERY
(2 × 3.7V)
R1
60.4k
+
R6
768k
R5
53.6k
V3
7.4V Li-Ion
BATTERY
(2 × 3.7V)
+
R7
140k
R4
113k
R10
768k
R11
140k
R9
53.6k
R8
113k
V1
UV1
OV1
V2
UV2
LTC4417
OV2
V3
UV3
OV3
HYS
RHYS
255k
1%
Figure 7. Configuring hysteresis voltages
output. Inrush current limiting circuitry
is not shown in Figure 5 for simplicity.
HIGH IMPEDANCE INPUT SOURCE
APPLICATIONS
Internal series resistance, present in all
batteries and capacitors, produces a
voltage drop that lowers the operating
voltage when load currents are present. Removal of the load current allows
the voltage source to recover this voltage drop. Some batteries and capacitors can recover hundreds of millivolts
when load currents are disconnected due
to a UV fault. If insufficient hysteresis
is provided, the input source can reenter its valid window and reconnect.
For these situations, the LTC4417 allows
the user to enable and set a hysteresis
current through an external resistor, RHYS .
When hysteresis is switched in, one-eighth
of the current flowing through RHYS flows
through the OV, UV resistive dividers to
generate the hysteresis voltage. By adjusting the value of the resistive dividers and/
or adding a resistor in series with the
OV and UV pins, individual hysteresis voltages can be tailored to each input source’s
internal resistance characteristic, preventing false reconnection after recovery.
Figure 7 shows a 255kΩ resistor,
RHYS, setting 245n A of hysteresis current through the resistive dividers, R1
through R3, to generate approximately
200mV of OV and UV hysteresis at the
12V wall adapter. Resistive T-structures,
R4 through R7 and R8 through R11,
are used to set independent OV and
July 2013 : LT Journal of Analog Innovation | 11
The LTC4417 can easily be cascaded to prioritize any number of input sources. Simply
connect all of the cascaded LTC4417s’ VOUT pins to the system output and connect
any higher priority LTC4417 CAS pin to the next lower priority LTC4417 EN pin.
UV hysteresis voltages of 201mV and
390mV, respectively, at the Li-ion batteries.
PRIORITIZE ANY NUMBER OF
SOURCES
The LTC4417 can easily be cascaded to
prioritize any number of input sources.
Simply connect all of the cascaded
LTC4417s’ VOUT pins to the system
output and connect any higher priority LTC4417 CAS pin to the next lower
Figure 8. Cascading application
priority LTC4417 EN pin as shown for
two LTC4417s in Figure 8. Additional
LTC4417 can be cascaded by daisy-chain
connecting their CAS and EN pins. Driving
the highest priority LTC4417 EN pin low
disconnects all input sources from the
common output. Driving the highest
priority LTC4417 SHDN pin low disables
that LTC4417 and allows the next LTC4417
in the serial chain to control the output.
IRF7324
M1
M2
The high voltage (2.5V to 36V), triple
input LTC4417 prioritized PowerPath
controller is easy to use, robust, and
automously allows applications to be
powered from a variety of input sources,
independent of voltage. Automatically
prioritized supply current sourcing
extends the life of lower priority input
sources while controlled switching
protects input sources from cross and
reverse conduction during switchover.
Key features such as continuous input
source monitoring through ±1.5%
accurate overvoltage and undervoltage
comparators, 256ms input deglitch time,
and strong gate drivers with an integrated
6.2V clamp, enable overvoltage, undervoltage, and reverse voltage protection
from faulty input source connections.
CVS1_1
0.1µF
VS1 G1
VOUT
LTC4417
MASTER
CONCLUSION
DISABLE ALL CHANNELS
SHDN MASTER
EN
SHDN
CAS
IRF7324
M3
M4
CVS1_2
0.1µF
Resistive divider defined overvoltage
and undervoltage trip points, adjustable current mode hysteresis, external
inrush current limiting, and external
back-to-back P-channel MOSFETs make
the LTC4417 customizable to numerous
applications. The LTC4417 is available
in 24-lead GN and leadless 4mm × 4mm
QFN packages. Both packages are available in C, I and –40ºC to 125ºC H grades.
VS1 G1
VOUT
LTC4417
SLAVE
12 | July 2013 : LT Journal of Analog Innovation
EN
SHDN
CAS
+
VOUT
CL
47µF
TO NEXT
LOWER PRIORITY
LTC4417
Visit www.linear.com/LTC4417 for
data sheets, demo boards and other
applications information. n
design features
3mm × 3mm QFN IC Directly Monitors 0V to 80V Supplies:
Features I2C Interface, Peak Value Tracking and Runs from
Any Supply
CY Lai
Power monitoring in combination with control mechanisms can significantly boost
system energy efficiency and reliability. The LTC2945 is a highly integrated digital
power monitoring solution that is compact, rugged and easy-to-use. It is designed to
fit applications requiring power monitoring with a minimal number of components.
MONITOR POWER ON ANY SUPPLY
•The secondary supply exists, but is not
readily accessible—it is inconveniently
located on the printed circuit board,
complicating the routing of a power line
The LTC2945’s internal current sense
amplifier features a common mode range
of 0V to 80V to suit a wide variety of high
side and low side current sensing applications. Most wide range supply monitors
available today require a low voltage
secondary supply for operation, which
can be undesirable for several reasons:
The LTC2945 avoids these problems by
integrating a high voltage linear regulator
that can be powered directly from 4V to
80V supplies. The output of the linear regulator (INTVCC) powers the LTC2945, and can
be externally bypassed to prevent supply
noise from corrupting the signal integrity
of internal circuitry. The linear regulator is
capable of supplying a 10m A load, saving
the cost of a dedicated high voltage linear
regulator needed to power circuits such
as opto-couplers in some applications.
•There is no suitable secondary supply
•The secondary supply, often loaded
with noisy digital circuits, must be sufficiently filtered or bypassed due to the
finite power supply rejection ratio of the
supply monitor at higher frequencies
Figure 1. Functional block
diagram of the LTC2945
SENSE+
VDD
SENSE–
ADR0
ADR1
ALERT
DECODER
+
VSTBY
20X
5.7V
VSTBY
GEN
INTV CC
2
IC
–
Figure 1 shows the LTC2945’s functional
block diagram. All basic elements required
for power monitoring are integrated,
including a precision current sense
amplifier, precision resistive dividers, an
analog-to-digital converter (ADC) and
an I2C interface for communicating with
the host controller. Only an external
current sense resistor is required. The
host can periodically poll the LTC2945
for available power data, minimum and
maximum values are stored, and an alert
can be sent from the LTC2945 to interrupt the host when measured values
exceed their preprogrammed limits.
735k
SDAO/SDAO
(LTC2945 / LTC2945-1)
VREF = 2.048V
735k
LOGIC
SDAI
VSTBY
12
MUX
6.3V
15k
6.4V
12-BIT ADC
SCL
REGISTERS
15k
6.4V
ADIN
GND
July 2013 : LT Journal of Analog Innovation | 13
The LTC2945 is a highly integrated power monitor that easily fits into
a wide range of systems. It offers a 0V to 80V common mode range,
2.7V to 80V operating range, ±0.75% accurate voltage and current
measurements, and an on-chip digital multiplier that computes power.
RSNS
VIN
4V TO 80V
SENSE+
RSNS
VIN
0V TO 80V
VOUT
SENSE+
SENSE–
VDD
SENSE–
SENSE+
LTC2945
VOUT
SENSE–
INTVCC
2.7V TO 5.9V
VDD
4V TO 80V
RSNS
VIN
0V TO 80V
VOUT
VDD
LTC2945
LTC2945
C2
INTVCC
C2
INTVCC
C2
GND
GND
Figure 2a. The LTC2945 deriving power
from the monitored supply
GND
Figure 2b. The LTC2945 deriving power
from a wide ranging secondary supply
Figure 2c. The LTC2945 deriving power
from a low voltage secondary supply
GND
RSNS
VIN
>80V
SENSE+
VOUT
SENSE+
SENSE–
>80V
INTVCC
VDD
RSNS
VIN
0V TO 80V
RSHUNT
RSHUNT
VOUT
SENSE–
VDD
VDD
INTVCC
VDD
INTVCC
LTC2945
GND
LTC2945
C1
GND
C2
INTVCC
C2
LTC2945
GND
LTC2945
C2
VNEG
>–80V
RSHUNT
Figure 3a. The LTC2945 deriving power
through a high side shunt regulator
Figure 3b. The LTC2945 deriving power
through a low side shunt regulator in a
high side current sense topology
Figure 2a shows a typical LTC2945 application monitoring a 4V to 80V supply and
deriving power off the same supply. The
bus voltage is measured at the SENSE+ pin
through an internal resistive divider and
a sense resistor is used to measure the
load current on the high side. If the bus
voltage to be monitored is below 2.7V,
the power for the LTC2945 can be derived
from a wide range secondary supply
14 | July 2013 : LT Journal of Analog Innovation
SENSE–
GND
GND
SENSE–
SENSE+
RSNS
VOUT
Figure 3c. The LTC2945 deriving power
through a low side shunt regulator in a
low side current sense topology
VNEG
–4V TO –80V
SENSE+
RSNS
VOUT
Figure 3d. The LTC2945 deriving
power from the monitored supply in
a low side current sense topology
as shown in Figure 2b or a low voltage
secondary supply as shown in Figure 2c.
standing off the bus voltage and supplying
LTC2945’s operating current will work.
The LTC2945 also integrates a 6.3V,
35m A shunt regulator at the INTVCC pin for
operation beyond 80V. Figure 3a shows
the LTC2945 used in one such application
with its ground floated at 6.3V below the
bus voltage. The bulk of the bus voltage is
dropped across an external shunt resistor;
in practice any current source capable of
Figure 4 shows how to measure the
bus voltage in this configuration using
a matched PNP pair and some resistors.
The resistor values shown are optimized
for VIN of 165V ±10%. The LTC2945’s
shunt regulator can also be configured
as shown in Figure 3b when the only
design features
LTC2945 integrates an oversampling ∆∑ ADC that inherently averages
the measured voltage over the conversion cycle to effectively reject
noise due to transient spikes and AC power line. Bus voltage,
sense voltage and ADIN are measured with total error of less than
±0.75% at full scale over the full industrial temperature range.
Figure 4. Application circuit for measuring
the bus voltage in a high side shunt
regulator configuration
±0.75% TOTAL ERROR
MEASUREMENT ACCURACY
VIN
R1
2k
R2
2k
NST30010MXV6
INTVCC
LTC2945 integrates an oversampling
DS ADC that inherently averages the
measured voltage over the conversion
cycle to effectively reject noise due to
transient spikes and AC power line harmonics. Bus voltage, sense voltage and
ADIN are measured with total error of
less than ±0.75% at full scale over the
full industrial temperature range.
LTC2945
ADIN
R3
1150k
R4
5.76k
GND
UP TO 8mA LOAD
(OPTOS, ETC.)
RSHUNT*
15k
*THREE 1W, 5k RESISTORS IN SERIES
secondary supply available exceeds 80V in
high side current sensing applications.
If the output of the power supply is
negative such as in –48V distributed
power systems for networking, communications and high end computing
equipment, low side current sensing is
generally preferred, as shown in Figures 3c
and 3d. Figure 3c shows a shunt resistor
and LTC2945’s shunt regulator limiting
INTVCC to 6.3V above a negative supply
that exceeds 80V. More commonly, the
negative supply is below 80V and instead
the internal linear regulator can be used to
power the LTC2945 directly, as shown
in Figure 3d. In this configuration
the VDD pin measures the bus voltage
through an internal resistive divider.
Measuring bus voltage in excess of
80V in low side current sensing applications such as Figure 3c can be done
by connecting a resistive divider to
the ADIN pin as shown in Figure 5.
Figure 5. ADIN senses the bus voltage in low side
shunt regulator configuration.
The 12-bit DS ADC provides a full-scale
voltage of 102.4mV (25µV/LSB) for sense
voltage, 102.4V (25mV/LSB) for bus voltage and 2.048V (0.5mV/LSB) for ADIN.
Typical integral linearity error (INL) of
the ADIN voltage and the sense voltage
are both well within ±0.5LSB, as shown in
Figures 6 and 7. The LTC2945 is also ideal
in applications where accuracy is important at the low end of the measurements
since its specified offset voltages are as
low as ±1.1LSB for ADIN and ±3.1LSB for
current sense voltage in the worst case.
Figure 6. ADIN INL curve
Figure 7. SENSE INL curve
0.3
0.2
GND
0.2
INTVCC
LTC2945
ADIN
R2
GND
0.1
ADC INL (LSB)
R1
ADC INL (LSB)
RSHUNT
VNEG > –80V
0.4
0.0
–0.1
0.0
–0.2
–0.2
–0.3
0
1024
2048
CODE
3072
4096
–0.4
0
1024
2048
CODE
3072
4096
July 2013 : LT Journal of Analog Innovation | 15
Opto-isolation is common in high voltage systems where the high
voltage sections must be galvanically isolated for safety reasons. The
LTC2945 accommodates isolated applications by splitting the SDA
signal on the I2C interface into an SDAI pin and an SDAO pin (for
LTC2945-1, SDAO) for applications with an opto-isolator interface.
PEAK VALUES TRACKING AND OVER/
UNDERVALUE ALERTS
the monitor needs to be monotonic and
of high resolution in order to minimize
stability issues. The LTC2945 generates
24-bit power data by digitally multiplying the 12-bit sense voltage and 12-bit bus
voltage data without truncating the result.
Keeping track of the minimum and maximum measurement values is important in
many power monitoring systems because
it could be used to study usage behavior
for more efficient resource allocation and
is often an indicator of system health.
Previously, gathering such information
required periodic polling of the power
monitor by the system’s microprocessor,
which wasted precious computation time
and potentially tied up the I2C interface.
The LTC2945 solves this problem by storing the minimum and maximum values
for power, voltage, current, and ADIN.
The Page Read feature on the LTC2945
allows these data to be read with just a
single I2C read transaction. An ALERT pin
can also be configured to signal overvalue or undervalue limit violations for
power, voltage, current and ADIN.
OPTO-ISOLATION AND SHUTDOWN
The LTC2945 can be shut down via the
serial I2C interface, reducing the typical
quiescent current to 20µ A—especially
important for battery-powered applications. Opto-isolation is common in high
voltage systems where the high voltage
sections must be galvanically isolated
for safety reasons. The LTC2945 accommodates isolated applications by splitting
the SDA signal on the I2C interface into an
SDAI pin and an SDAO pin (for LTC2945-1,
SDAO) for applications with an optoisolator interface as shown in Figure 8.
For limited amounts of current, the
internal linear regulator or shunt regulator can be used to supply the pull-up
resistors on the I2C bus. In situations
where it is undesirable to tap off the
internal regulator and a low voltage
UNTRUNCATED 24-BIT POWER DATA
For applications where a digital servo loop
is used to regulate the power output of a
system, the power data read back from
Figure 8. Opto-isolation of a 10kHz I2C interface between the LTC2945 and a
microcontroller
supply is not available, the LTC2945-1
allows the pull-up resistors to connect
directly to high voltages. The SCL and the
SDAI pins are limited to safe voltages by
internal 6.3V, 3m A clamps. The SDAO pin
is inverted (to SDAO) so that it can be
clamped by the anode of the input diode
of an optoisolator as shown in Figure 9.
SUPPLY TRANSIENTS
The wide operating range of the LTC2945 is
advantageous even in applications where
the bus voltage is normally well below
80V. Transient voltage surges due to
inductive kickbacks in automotive load
dump situations and hot swap output
shorts are just two possible scenarios
where a rugged power monitoring solution is required in order to withstand
overvoltage conditions far in excess
of the normal operating voltage.
The 100V absolute maximum rating
of the LTC2945 makes it easy to guard
against these types of voltage surges since
there is a wide range of transient surge
suppressor (TVS) diodes from which to
choose. In certain applications a large
Figure 9 Opto-isolation of a 1.5kHz I2C interface between the LTC2945-1 and a
microcontroller
3.3V
3.3V
5V
SDAI
R4
10k
R5
0.82k
R6
0.51k
48V
R7
10k
R4
20k
µP
1/2 MOCD207M
LTC2945-1
GND
SDA
SDAO
GND
SDAO
16 | July 2013 : LT Journal of Analog Innovation
µP
1/2 MOCD207M
GND
1/2 MOCD207M
R7
10k
VDD
SDA
GND
R6
0.51k
SDAI
VDD
LTC2945
R5
7.5k
1/2 MOCD207M
design features
The 100V absolute maximum rating of the LTC2945 makes it easy to guard against voltage
surges since there is a wide range of transient surge suppressor (TVS) diodes from which
to choose. In certain applications a large MOSFET power device can break down to clip the
inductive spike safely, and in most 12V and 24V systems the break-down voltage of these
power devices is less than 100V, potentially negating the requirement for a TVS diode.
RSNS
0.02Ω
VIN
SENSE +
VOUT
SENSE–
INTVCC
VDD
R12
100Ω
R4
1k
LTC2945
Q1
PZTA42
R3
0.51k
R1
1k
C2
1µF
R2
1k
FGND
3.3V
R7
0.51k
R8
0.51k
R9
1k
R10
1k
R11
10k
VDD
VCC
T1
SMAJ78A
SCL
C1
0.1µF
SCL
SDAI
SDA
GND
HCPL-063L
FGND
µP
3.3V
VCC
ADR1
ADR0
SDAO
ADIN
INT
ALERT
GND
GND
GND
HCPL-063L
FGND
M1
BSP149
Figure 10. Ruggedized 4V to 80V high side power
monitor with surge protection up to 200V
MOSFET power device can break down
to clip the inductive spike safely, and
in most 12V and 24V systems the breakdown voltage of these power devices
is less than 100V, potentially negating the requirement for a TVS diode.
Hard-clamping the voltage with TVS diode
or MOSFET break down may not be practical when the inductive energy is too
high or unpredictable. Figure 10 shows
a LTC2945-based power monitor that can
ride through a 200V surge where its high
voltage pins are clamped by T1 to less than
80V. In normal operation, M1 operates in
the triode region with the device ground
R5
1Ω
at a few mV above the system ground.
During the surge, the device ground is
lifted by T1 and the balance of the surge
voltage is dropped across M1. The BSP149
has a 200V break down and the surge
duration is limited by its safe operating
area—for example at room temperature it
can survive a 200V surge for 1ms at VIN .
multiplier that computes power. Digital
watchdog functions such as peak and valley values and window comparators are
available for power, voltage, current and
an external voltage. Opto-isolation is simplified with a split SDA pin. The LTC2945
is available in space-saving 3mm × 3mm
QFN and 12-pin MSOP packages.
CONCLUSION
Visit www.linear.com/LTC2945 for
data sheets, demo boards and other
applications information. n
The LTC2945 is a highly integrated power
monitor that easily fits into a wide range
of systems. It offers a 0V to 80V common
mode range, 2.7V to 80V operating range,
±0.75% accurate voltage and current
measurements, and an on-chip digital
July 2013 : LT Journal of Analog Innovation | 17
High Efficiency Switching Power Conversion Combined with
Low Noise Linear Regulation in µModule Package
David Ng
Devices with high speed
or high resolution functions
require clean power.
Switching regulators offer
efficiency across a variety
of input/output conditions,
but the typical switcher is
hard pressed to deliver the
clean, low output noise
and fast transient response
needed by high data rate
FPGA I/O channels or high
bit count data converters. In
contrast, high performance
linear regulators feature
low output noise and fast
transient response, but
can quickly heat up.
Figure 2. In a 12V input to 1.2V output, 5A
application, the LTM8028 dissipates less than 4W
and heats up by only 45°C.
POWER LOSS (W)
4
40
3
30
TEMP RISE
2
20
POWER LOSS
1
0
10
0
1
LTM8028
VIN
402k
10µF
0.01µF
RUN
MARGA
IMAX
BKV
SS
165k
RT
VOUT
1.2V
5A
VOUT
LINEAR
REGULATOR SENSEP
PGOOD
SYNC
GND
VOB VO0 VO1 VO2
37µF*
+
100µF
470µF
*37µF = 4.7µF + 10µF + 22µF IN PARALLEL
Figure 1. The LTM8028 is a 36V input, UltraFast, low output noise 5A µModule regulator.
The LTM®8028 combines the best of
both worlds—a high efficiency synchronous switching converter controlled by
an UltraFast™ linear regulator, both
integrated into a small, 15mm × 15mm
µModule® package. It is available in
LGA (4.32mm tall) and BGA (4.92mm
tall) lead styles, both of which are
RoHS compliant.
The linear regulator controls the output of
the switcher to 300mV above the desired
output voltage to provide the optimum
combination of headroom, efficiency
and transient response. The LTM8028
accepts inputs as high as 40V and produces output voltages between 0.8V and
1.8V at up to 5A. A typical 1.2V output
application is shown in Figure 1.
Figure 3. At 1.0V output, the LTM8028 transient
response is less than 20mV.
Figure 4. The LTM8028 transient response is only
38mV.
50
VIN = 12V
VOUT = 1.2V
2
3
ILOAD (mA)
4
18 | July 2013 : LT Journal of Analog Innovation
5
0
TEMPERATURE RISE (°C)
5
VIN
6V TO 36V
VOUT
20mV/DIV
20mV
IOUT
2A/DIV
38mV
VOUT
20mV/DIV
IOUT
2A/DIV
10µs/DIV
VOUT = 1V
COUT = 100µF + 22µF + 10µF + 4.7µF
LOAD STEP = 0.5A TO 5A
tRISE/FALL = 1µs
20µs/DIV
VOUT = 1.8V
COUT = 100µF + 22µF + 10µF + 4.7µF
LOAD STEP = 0.5A TO 5A
tRISE/FALL = 1µs
design features
The linear regulator controls the output of the switcher to 300mV above the
desired output voltage to provide the optimum combination of headroom,
efficiency and transient response. The LTM8028 accepts inputs as high as
40V and produces output voltages between 0.8V and 1.8V at up to 5A.
VIN
9V TO 15V
LTM8028
VIN
150k
10µF
0.01µF
RUN
MARGA
IMAX
BKV
SS
FULL LOAD
NOISE
AND RIPPLE
500µV/DIV
1mV
82.5k
RT
VOUT
1.2V
5A
VOUT
LINEAR
REGULATOR SENSEP
PGOOD
SYNC
GND
VOB VO0 VO1 VO2
f = 500kHz
137µF*
+
100µF
470µF
*137µF = 100µF + 4.7µF + 10µF + 22µF IN PARALLEL
1µs/DIV
Figure 5. The peak-to-peak switching noise at the output of the LTM8028 is less than 1mV. (Schematic shows the setup used to achieve these results.)
The output voltage of the LTM8028 is
set by controlling three 3-state inputs,
VO0, VO1 and VO2 . Applying a voltage to
the MARGA pin allows the user to margin
the output by as much as ±10%. The
current limit may be reduced from the
5A maximum through the IMAX pin, and
a PGOOD signal indicates that the output
is within 10% of the target voltage.
Figure 6. The output noise spectral density, peaking
at only 4μV/√Hz, makes the LTM8028 a good
candidate for sensitive data conversion circuits.
10
µV/√Hz
1
0.1
0.01
0.001
COUT = 137µF
VOUT = 1.8V
IOUT = 5A
VIN = 12V
10
100
1k
10k
FREQUENCY (Hz)
100k
1M
A design using a traditional linear regulator providing 1.2V at 5A from a 12V source
would burn over 50W and might require
expensive heat sinking. The LTM8028, as
shown in Figure 2, dissipates a twelfth of
that, less at 4W, yielding a typical junction temperature rise of only 45°C.
The heart of the LTM8028 is the high
performance linear regulator. Its total
line and load regulation below 0.2% at
room temp and 1% over its full –40°C to
125°C temperature range. Its UltraFast
bandwidth gives the LTM8028 a 10%–90%
load step transient response of only
2%. Figures 3 and 4 show the transient
response of the LTM8028 when the load
steps from 0.5A to 5A at a slew rate of
1A /µs when the device is configured
to deliver 1V and 1.8V, respectively.
Even though the linear regulator and
the synchronous switching converter are
packaged together, high power supply
rejection and integrated noise mitigation result in low output noise. Figure 5
shows peak-to-peak noise less than 1mV.1
In the frequency domain, the spectral noise
content is very low, peaking at 4µV/√Hz
at the switching converter’s fundamental
frequency of 300kHz as shown in Figure 6.
This is important when powering high
bit count data conversion circuits.
CONCLUSION
When a system design requires low
power loss, tight regulation, fast transient
response, and low output noise, reach
for the LTM8028 µModule regulator. It
combines the best features of high performance switching and linear regulators
into a single, space efficient package.
Visit www.linear.com/LTM8028 for
data sheets, demo boards and other
applications information. n
Notes
1Measuring low amplitude noise can be tricky. This
measurement was made using coaxial cables, impedance matching and a 150MHz HP461A amplifier. This
is similar to the setup described in Linear Technology
Application Note 70, “A Monolithic Switching Regulator
with 100μV Output Noise” by Jim Williams, except the
measurement here is not bandwidth limited to 10MHz.
July 2013 : LT Journal of Analog Innovation | 19
µModule Regulator Powers and Protects Low Voltage
µProcessors, ASICs and FPGAs from Intermediate Bus
Voltage Surges
Willie Chan and Jason Sekanina
Intermediate bus voltages of 24V~28V nominal are
commonplace in industrial, aerospace and defense
systems where series-connected batteries may be a
backup power source and 12V bus architectures tend to
be impractical due to distribution losses. The widening
voltage gap between the system bus and the power
inputs of digital processors present design challenges
relating to power delivery, safety and solution size.
If a single-stage non-isolated step-down
DC/DC converter is used at the point of
load, it must operate with extremely
accurate PFM/PWM timing. Input surge
events can stress DC/DC converters,
presenting an overvoltage risk to the
load. Erroneous or counterfeit capacitors introduced in manufacturing may
cause output voltage excursions exceeding
the load’s ratings potentially causing the
FPGA, ASIC or microprocessor to ignite.
Depending on the extent of the damage,
the root cause may be difficult to find.
An overvoltage risk mitigation plan is
absolutely necessary to prevent customer
dissastifaction. Traditional overvoltage
protection schemes involving a fuse are
not necessarily fast enough, nor dependable enough, to protect modern FPGAs,
ASICs and microprocessors, particularly
when the upstream voltage rail is 24V or
Bus voltage surges, which are often
present in aerospace and defense applications, pose a danger not only to the
DC/DC converter, but also to the load.
The DC/DC converter must be rated to
regulate through the overvoltage surge
with a fast control loop, so that sufficient line rejection is achieved.
28V nominal. Active protection at the
POL DC/DC is necessary. The LTM4641
is a 38V-rated, 10A DC/DC step-down
µModule® regulator that defends
against, and recovers from, many
faults, including output overvoltage.
IMPORTANCE OF ACCURATE
SWITCHER TIMING INCREASES WITH
INPUT VOLTAGE & SURGES
When a wide differential exists between
the input and the output voltages, switching DC/DC regulators are favored over
linear regulators for their much higher
efficiency. To achieve a small solution size,
a nonisolated step-down converter is the
top choice, operating at high enough frequency to shrink the size requirements of
its power magnetics and filter capacitors.
If the DC/DC converter fails to regulate
or survive a bus surge, an overvoltage is
presented to the load. Overvoltage faults
may also be introduced as the load’s
bypass capacitors degrade with age and
temperature, which results in looser
transient load response over the course
of the end product’s life. If the capacitors
degrade beyond the limits of the control
loop’s design, the load can be exposed to
overvoltage by two possible mechanisms:
However, in high step-down ratio applications, a DC/DC switching converter
must operate at duty-cycles down to 3%,
Figure 1. Traditional overvoltage protection circuit consisting of a fuse, SCR and
Zener diode. While inexpensive, this circuit’s response time is insufficient to reliably
protect the latest digital circuits, particularly when the upstream supply rail is an
intermediate voltage bus. Moreover, recovery from an overvoltage fault is invasive
and time consuming.
20 | July 2013 : LT Journal of Analog Innovation
demanding accurate PWM/PFM timing.
Furthermore, tight voltage regulation is
required by digital processors, and fast
transient response is needed to keep the
voltage within safe limits. At relatively
high input voltages, the margin for error
in the on-time of the top side switch
of the DC/DC regulator is reduced.
POWER SUPPLY REGULATED LINE
DC/DC
+
CIN(BULK)
SCR
CMLCC
LOAD
design features
The LTM4641 is a 38V-rated, 10A DC/DC step-down
µModule switching regulator that defends against, and
recovers from, many faults, including output overvoltage.
4
SHORT-CIRCUIT APPLIED
VINL, VINH
25V/DIV
CROWBAR
5V/DIV
VOUT
200mV/DIV
4
3
2
VIN
4V TO 38V
4.5V START-UP
+
MSP*
10µF
50V
×2
100µF
50V
3
VING VINGP VINH MTOP
VOUT
VINL
1.1V VOUT PEAK
750k
see this circuit in action at
video.linear.com/143
1
MBOT
VOSNS+
LTM4641
RUN
TRACK/SS
10nF
2
MCB**
CROWBAR
fSET
UVLO
INTVCC
DRVCC
1
SW
VOSNS–
COUT
100µF
×3
VOUT
1V
10A
5.49k
5.49k
LOAD
GND
OVPGM
IOVRETRY OVLO FCB LATCH SGND
5.6M
4µs/DIV
VIN = 38V
VOUT = 1V
OVERVOLTAGE TRIP THRESHOLD SET TO 5%
Figure 2. The LTM4641 responds to an overvoltage
condition within 500ns, protecting the load from
voltage stress.
•First, even if the control loop
remains stable, heavy transient
load-step events will demonstrate
higher voltage excursions than were
expected at the onset of design.
•Second, if the control loop becomes
conditionally stable (or, worse yet,
unstable), the output voltage can
oscillate with peaks exceeding acceptable limits. Capacitors can also
degrade unexpectedly or prematurely
when an incorrect dielectric material is used, or when fake components enter the manufacturing flow.
CHEAP COUNTERFEIT COMPONENTS
GENERATE EXPENSIVE HEADACHES
Gray market or black market counterfeit components can be enticing, but
they don’t meet the standards of the
genuine article (e.g., they may be recycled,
reclaimed from electronic waste, or
SGND CONNECTS TO GND INTERNAL TO µMODULE REGULATOR
* MSP: (OPTIONAL) SERIES-PASS OVERVOLTAGE POWER INTERRUPT MOSFET, NXP PSMN014-60LS
** MCB: (OPTIONAL) OUTPUT OVERVOLTAGE CROWBAR MOSFET, NXP PH2625L
Figure 3. LTM4641 output overvoltage protection plan. The
probe icons correspond to the waveforms in Figure 2.
built from inferior materials). A shortterm savings becomes a huge long-term
expense when a counterfeit product fails.
Counterfeit capacitors, for example, can
fail in a number of ways. Counterfeit
tantalum capacitors have been seen to
suffer internal self-heating with a positivefeedback mechanism to the point of reaching thermal runaway. Counterfeit ceramic
capacitors may contain compromised or
inferior dielectric material, resulting in an
accelerated loss of capacitance with age
or at elevated operating temperatures.
When capacitors fail catastrophically or
degrade in value to induce control loop
instability, the voltage waveforms can
become much greater in amplitude than
originally designed, endangering the load.
Unfortunately for the industry, counterfeit components are increasingly finding their way into the supply chain and
electronics manufacturing flow, even in
the most sensitive and secure applications.
A United States Senate Armed Services
Committee (SASC) report released publicly
in May 2012 found widespread counterfeit electronic components in military
aircraft and weapon systems that could
compromise their performance and reliability—systems built by the top contractors in the defense industry. Coupled
with the increasing number of electronic
components in such systems—more than
3,500 integrated circuits in the new Joint
Strike Fighter—counterfeit components
pose a system performance and reliability risk that can no longer be ignored.
RISK MITIGATION PLANNING
Any risk mitigation plan should consider
how the system would respond to and
recover from an overvoltage condition. Is
the possibility of smoke or fire resulting
July 2013 : LT Journal of Analog Innovation | 21
R1
20k
see this circuit in action at
video.linear.com/148
4.5V START-UP
OPERATION UP TO 28VIN
CONTINUOUS, TRANSIENT
PROTECTED TO 80VIN
+
CIN(BULK)
100µF
100V
RT1
NTC
MSP
CIN(MLCC)
10µF
100V
×2
5V
D1
36V
2%
R2
8.25k
RfSET
1M
VINL
VING VINGP
D2
RROV
4.7M
R3
2.7M
OUT
LT3010-5
SENSE
SHDN
GND
LATCH
VORB+
HYST
FCB
VOSNS+
LTM4641
VOSNS–
VORB–
TEMP
1VREF
OVPGM
OTBH
PGOOD
INTVCC
DRVCC
IOVRETRY
RUN
TRACK/SS
TMR
CSS
1nF
D2: CENTRAL SEMI CMMSH1-100G
MCB: NXP PSMN5R0-30YL
MSP: NXP PSMN028-100YS
RT1: MURATA NCP15WM474J03RC
MCB
CROWBAR
OVLO
RBOV
29.4k
COMP SGND
5V
100µF
6.3V
×4
VOUT
1V
10A
RSET1A
5.49k
RSET1B
5.49k
LOAD
LOCAL HIGH
FREQUENCY
DECOUPLING
GND
CTMR
1µF
SGND CONNECTS TO GND INTERNAL TO MODULE. KEEP SGND
ROUTES/PLANES SEPARATE FROM GND ON MOTHERBOARD
from an overvoltage fault acceptable?
Would efforts to determine root cause and
implement corrective actions be hampered
by damage resulting from an overvoltage
fault? If a local operator were to powercycle (reboot) a compromised system,
would even greater harm to the system
result further hindering recovery efforts?
What is the process and time required
to determine the cause of the fault and
resume normal system operation?
•Variations in the Zener diode
breakdown voltage, SCR gate trigger threshold, and current required
to blow the fuse result in inconsistent response times. Protection may
engage too late to prevent hazardous voltage from reaching the load.
INADEQUACIES OF TRADITIONAL
PROTECTION CIRCUIT
•If the voltage rail under consideration powers the digital core, an SCR’s
protection capability is limited, since
the forward drop at high currents is
comparable to or above the core voltage of the latest digital processors.
22 | July 2013 : LT Journal of Analog Innovation
VOUT
UVLO
VIN
This straightforward circuit is relatively simple and inexpensive, but there
are drawbacks of this approach:
SW
fSET
Figure 4. Input surge protection up
to 80V, using the LTM4641 and an
external LDO
A traditional overvoltage protection
scheme consists of a fuse, silicon controlled rectifier (SCR), and Zener diode
(Figure 1). If the input supply voltage
exceeds the Zener breakdown voltage,
the SCR activates, drawing sufficient current to blow open the upstream fuse.
VINH
SWITCHING ACTION IS
TEMPORARILY LATCHED OFF IF
VIN EXCEEDS 80V; AUTONOMOUS
RESTART ATTEMPS OCCUR IN
9 SECOND INTERVALS WHEN INPUT
VOLTAGE RETURNS BELOW 80V. NOTE
LT3010-5 IS RATED FOR 80V, ABSOLUTE
MAXIMUM.
•The level of effort required to
recover from a fault is high, involving physically servicing the fuse
and restarting the system.
Because of these drawbacks, the traditional overvoltage protection scheme is
not suitable for high voltage to low voltage DC/DC conversion powering loads such
as ASICs or FPGAs that could be valued in
the hundreds if not thousands of dollars.
COMBINE POWER AND FAULT
PROTECTION FOR FAST AND
DEPENDABLE REACTION AND
RECOVERY
A better solution would be to accurately
detect an imminent overvoltage condition and respond by quickly disconnecting the input supply while discharging
excess voltage at the load with a low
impedance path. This is possible with
the protection features in the LTM4641.
At the heart of the device is a 38V-rated,
10A step-down regulator with the inductor, control IC, power switches and
compensation all contained in one
surface mount package. It also includes
extensive monitoring and protection
circuitry to protect high value loads such
as ASICs, FPGAs and microprocessors. The
LTM4641 maintains a constant watch for
input undervoltage, input overvoltage,
overtemperature and output overvoltage and overcurrent conditions and
acts appropriately to protect the load.
design features
Overvoltage protection in mission critical systems must be fast, accurate
and consistent—at levels beyond the capabilities of the traditional SCR/fusebased scheme. The LTM4641 combines an efficient 10A DC/DC step-down
regulator with a fast and accurate output overvoltage protection circuit in one
surface mount package to meet demanding protection requirements.
To avoid false or premature execution
of the protection features, each of these
monitored parameters has built-in glitch
immunity and user adjustable trigger
thresholds with the exception of overcurrent protection, which is implemented
reliably, cycle-by-cycle with current-mode
control. In the case of an output overvoltage condition, the LTM4641 reacts within
500ns of fault detection (Figure 2).
The LTM4641 responds nimbly and reliably
to protect downstream devices, and, unlike
fuse-based solutions, it can automatically
reset and rearm itself after fault conditions
have subsided. The LTM4641 uses an internal differential sense amplifier to regulate
the voltage at the load’s power terminals, minimizing errors stemming from
common-mode noise and PCB trace voltage
drops between the LTM4641 and the load.
The DC voltage at the load is regulated to
better than ±1.5% accuracy over line, load
and temperature. This accurate output
voltage measurement is also fed to the fast
output overvoltage comparator, which
triggers the LTM4641’s protection features.
When an overvoltage condition is detected,
the µModule regulator rapidly initiates
several simultaneous courses of action.
An external MOSFET (MSP in Figure 3)
disconnects the input supply, removing
the high voltage path from the regulator
and the high value load. Another external
MOSFET (MCB in Figure 3) implements a
low impedance crowbar function, quickly
discharging the load’s bypass capacitors
(COUT in Figure 3). The LTM4641’s builtin DC/DC step-down regulator enters a
latched-off shutdown state and issues
a fault signal indicated by the HYST pin
which can be used by the system to initiate a well managed shutdown sequence
and/or system reset. A dedicated voltage reference independent of the control
loop’s reference voltage is used to detect
fault conditions. This provides resilience
against a single-point failure, should the
control loop’s reference happen to fail.
The LTM4641’s protection features are
bolstered by its fault recovery options. In
a traditional overvoltage fuse/SCR protection scheme, a fuse is relied upon to
separate the power supply from the high
value load. Recovery from a fuse-blowing
fault requires human intervention—someone with physical access to the fuse to
remove and replace it—introducing an
unacceptable delay in fault recovery for
high uptime or remote systems. In contrast, the LTM4641 can resume normal
operation once the fault condition has
cleared either by toggling a logic level
control pin or by configuring the LTM4641
for autonomous restart after a specified
timeout period. If fault conditions reappear after the LTM4641 resumes operation,
the aforementioned protections immediately re-engage to protect the load.
INPUT SURGE PROTECTION
In some cases, output overvoltage protection alone is insufficient, and input
overvoltage protection is required. The
LTM4641’s protection circuitry can monitor
the input voltage and activate its protection features should a user-configured
voltage threshold be exceeded. If the
anticipated maximum input voltage
exceeds the 38V rating of the module,
input surge protection can be extended
up to 80V with the LTM4641 still fully
operational by adding an external high
voltage LDO to keep control and protection circuitry alive (Figure 4).
CONCLUSION
Mission critical electronics increasingly mix a distributed power bus in the
range of 12V–28V with low voltage high
performance digital ICs. Risk mitigation has become more important than
ever, particularly when the power bus is
susceptible to voltage surges. The latest,
costly FPGAs, ASICs, and microprocessors
demand supply voltages with an absolute
maximum limit as low as 3%–10% of the
nominal voltage, making them extremely
susceptible to damage, even fire from an
overvoltage fault. Faults can be caused by
timing errors in the switching regulator, an
input voltage surge or improper components introduced during manufacture.
Overvoltage protection in mission critical
systems must be fast, accurate and consistent—at levels beyond the capabilities
of the traditional SCR/fuse-based scheme.
The LTM4641 combines an efficient
10A DC/DC step-down regulator with a fast
and accurate output overvoltage protection circuit in one surface mount package
to meet demanding sytem requirements.
Visit www.linear.com/LTM4641 for
data sheets, demo boards and other
applications information. n
Acknowledgements
•Afshin Odabaee, Product Marketing Manager, µModule
Power Products, Linear Technology
•Yan Liang, Applications Engineer, Linear Technology
July 2013 : LT Journal of Analog Innovation | 23
What’s New with LTspice IV?
Gabino Alonso
video.linear.com/167
Follow @LTspice on Twitter for
up-to-date information on models, demo circuits,
events and user tips: www.twitter.com/LTspice
VIDEOS
Buck Regulators
Noise Simulations
(video.linear.com/167)
• LTC3838-2: High efficiency step-down
DC/DC converter with DCR current
sensing (4.5V–14V to 0.4V–2.5V at 50A)
www.linear.com/LTC3838-2
Tyler Hutchison, Applications Engineer,
shows how to setup an LTspice®
.noise simulation to view both input
and output referred voltage noise
and discusses a couple of tricks to
determining noise contributors.
SELECTED DEMO CIRCUITS
Boost & SEPIC Regulators
• LT3957A: High efficiency output boost
converter (4.5V–16V to 24V at 600m A)
www.linear.com/LT3957A
• LTC3122: 5V to 12V synchronous
boost converter with output
disconnect (1.8V–5.5V to 12V at 0.8A)
www.linear.com/LTC3122
• LTM8001: Two output regulator with
supercapacitor backup power
(9V–15V to 3.3V at 1A & 2.5V at 0.5A)
www.linear.com/LTM8001
Battery Chargers & LED Drivers
• LT3651-8.4: 4A, 2-cell charger
with maximum power point
control (16V–32V to 8.4V at 4A)
www.linear.com/LT3651-8.2
• LT3763: 20A, pulse-width modulated,
single LED driver (10V–30V to 6V LED at
20A) www.linear.com/LT3763
Cable Drop Compensators
• LTC3890-2: Automotive 12V SEPIC
and 3.3V step-down converter
(5V–35V to 12V at 2A and 3.3V at 10A)
www.linear.com/LTC3890-2
• LT6110: Wire loss compensation using a
current referenced LDO (4.9V–15V to 3V at
1A) www.linear.com/LT6110
Buck-Boost Regulators
• LTC4417: Priority switching from
12V main to 14.8V battery backup supply
www.linear.com/LTC4417
• LT8705: Telecom voltage stabilizer
(36V–80V to 48V at 5A)
www.linear.com/LT8705
PowerPath Controllers
What is LTspice IV?
LTspice® IV is a high performance SPICE simulator, schematic capture and waveform viewer designed to speed
the process of power supply design. LTspice IV adds enhancements and models to SPICE, significantly reducing
simulation time compared to typical SPICE simulators, allowing one to view waveforms for most switching
regulators in minutes compared to hours for other SPICE simulators.
LTspice IV is available free from Linear Technology at www.linear.com/LTspice. Included in the download is a
complete working version of LTspice IV, macro models for Linear Technology’s power products, over 200 op amp
models, as well as models for resistors, transistors and MOSFETs.
24 | July 2013 : LT Journal of Analog Innovation
SELECTED MODELS
Buck-Boost Regulators
• LTC3129: 15V, 200m A synchronous
buck-boost DC/DC converter
with 1.3µa quiescent current
www.linear.com/LTC3129
• LTC3245: Wide VIN range, low noise,
250m A buck-boost charge pump
www.linear.com/LTC3245
Buck Regulators
• LTC3621/-2: 17V, 1A synchronous stepdown regulator with 3.5µ A quiescent
current www.linear.com/LTC3621
• LTC3639: High efficiency,
150V 100m A synchronous step-down
regulator www.linear.com/LTC3639
• LTC3838-1/-2: Dual, fast, accurate
step-down DC/DC controller with
dual differential output sensing
www.linear.com/LTC3838-1
• LTC3883/-1: Single phase step-down
DC/DC controller with digital
power system management
www.linear.com/LTC3883
Inverting & SEPIC Regulator
• LTM8045: Inverting or SEPIC
µModule DC/DC converter with
up to 700m A output current
www.linear.com/LTM8045
design ideas
Battery Chargers
Hot Swap™ & PowerPath Controllers
Oscillators
• LT3651-8.2/8.4: Monolithic 4A high
voltage 2-cell Li-ion battery charger
www.linear.com/product/LT3651-8.2
• LTC4226: Wide operating range
dual Hot Swap controller
www.linear.com/LTC4226
• LTC6995-1/2: TimerBlox®: long
timer, low frequency oscillator
www.linear.com/LTC6995 n
Cable Drop Compensators
• LTC4417: Prioritized PowerPath controller
www.linear.com/LTC4417
• LT6110: Cable/wire drop compensator
www.linear.com/LT6110
Power User Tip
INTEGRATING NOISE OVER A BANDWIDTH
LTspice IV can perform .noise analysis of a circuit, where the noise voltage density (V/√Hz) for 1Hz bandwidth) can easily be plotted for the output noise, input noise or for
any noisy component like a resistor, diode or transistor. (To learn more about how to perform a .noise simulation, check out Tyler Hutchison’s noise simulation video at
http://video.linear.com/167.)
LTspice can also integrate noise over
bandwidth. To use this feature, Ctrlclick the data trace label, V(noise),
in the waveform viewer—the total
RMS noise based on the bandwidth
specified in the .noise directive is
displayed.
If you prefer to calculate the total
noise over a limited bandwidth rather
than the bandwidth specified in the
.noise directive you can modify the
left (low) and right (high) frequency
limits of the waveform viewer by
clicking on the horizontal axis. Once
you have your waveform displaying
the bandwidth of interest, Ctrl-click
the data trace label to display
the total RMS noise based in the
bandwidth of interest.
Alternately, you can add a .measure
statement to your schematic that
will calculate the total noise over a
particular bandwidth, and view your
results in the LTspice error log (Ctrl
+ L) after each simulation. (More
information on using .measure
statement can be found in LTspice
Help (F1) and in the video at
video.linear.com/115.)
Add a .measure statement
to your scheme to produce
calculated noise in the error log.
Happy simulations!
July 2013 : LT Journal of Analog Innovation | 25
Reference Clock Distribution for a 325MHz IF
Sampling System with over 30MHz Bandwidth,
64dB SNR and 80dB SFDR
Michel Azarian
Clock jitter introduced in an RF receiver through reference
clock buffering and distribution can limit achievable system
performance. The low jitter requirement is further exaggerated
when a relatively high intermediate frequency (IF) is used
in an effort to reap the benefits of relaxed front-end filter
requirements. This article details the design of a 325MHz
IF sampling system and introduces a clock buffer and
distributor that converts a sine wave reference signal into a
pair of differential LVPECL clocks appropriate to drive high
speed ADCs, and does so while minimizing introduced jitter.
SYSTEM DESCRIPTION
In an IF sampling (or undersampling)
system, where the ADC performs the last
stage of downconversion in an RF receiver,
the higher the IF is, the less steep the
image rejection filter in the RF front-end
can be. This helps in reducing the filter cost, size and insertion loss, which
further lessens the need for amplification,
leading to even lower cost and power
consumption. Figure 1 shows a typical
RF receiver chain employing IF sampling.
The downside of designing a receiver
with a relatively high IF is that system
specifications become more susceptible
to the degraded ADC performance while
sampling a higher-frequency analog
input signal. ADC spurious free dynamic
range (SFDR), for instance, worsens
Figure 1. A typical single-IF-stage
RF receiver block diagram
with higher input frequencies. More
importantly, the ADC aperture jitter
and its clock jitter, combined, begin to
define the achieved signal-to-noise ratio
(SNR) while sampling faster inputs.
The effect of clock jitter can be demonstrated by comparing the voltage error
magnitudes due to clock jitter while
sampling two slewing signals, one with a
higher slope than the other, using the same
ADC and clock. This clock has the same
amount of time jitter (tJ in s-RMS) while
sampling the two signals as illustrated in
Figure 2. The amount of uncertainty introduced due to clock jitter is indeed higher
with the faster moving signal, and, hence,
clock jitter is a major, if not dominant,
error source limiting the SNR when the
analog input has higher frequency content.
To avoid AM-to-PM noise conversion at the
ADC clock input, the clock should have a
high slew rate, ideally be a square wave.
The clock input of the ADC performs the
role of a limiter, taking in a signal and
squaring it by making decisions at the
input signal’s zero (or some other reference) crossings. AM-to-PM noise conversion occurs when the incoming signal has
a slow slew rate, like in a low frequency
and/or amplitude sine wave, where the
signal goes through the zero crossings
what resembles slow motion as compared to a square wave. If there is any
type of AM noise, for example, resistor
thermal noise, coupled noise from the
power supplies, etc., the zero crossings
of the incoming signal become inconsistent between consequent edges, leading
to the creation of jitter at the limiter’s
output; thus, AM noise is converted into
PM noise. Whereas, and if the incoming
signal rushes through the zero crossing, as
an LVPECL signal would normally do due
to its fast rise and fall times, the AM noise
added to the clock has no or very little
chance of being converted into PM noise.
ANTENNA
IF SAMPLING
DOWN-CONVERTING
MIXER
BPF
BAND
SELECTIVITY
FILTER
26 | July 2013 : LT Journal of Analog Innovation
Therefore, it is fundamental to keep the jitter of the ADC clock, denoted as the IF sampling clock in Figure 1, as low as possible.
LNA
BPF
BPF
IMAGE
REJECTION
FILTER
IF
SELECTIVITY
FILTER
LO
ADC
IF SAMPLING
CLOCK
DIGITAL PROCESSOR
design ideas
The downside of designing a receiver with a relatively high IF is that system
specifications become more susceptible to the degraded ADC performance
while sampling a higher-frequency analog input signal. ADC spurious free
dynamic range (SFDR), for instance, worsens with higher input frequencies.
More importantly, the ADC aperture jitter and its clock jitter, combined, begin to
define the achieved signal-to-noise ratio (SNR) while sampling faster inputs.
Figure 2. Effect of clock jitter while
digitizing slow and fast slewing signals
SLOW INPUT SIGNAL
FAST INPUT SIGNAL
VERROR(FAST)
VERROR(SLOW)
tJ
Also, most modern ADCs require their
clock input to be driven differentially to
attain their optimal performance. The
clock signals are commonly routed on the
PCB for quite some distance since their
source and destination are usually not
located close to each other. Running the
clock signals in differential form makes
them immune to coupling and leads to
an overall more robust design as compared to single-ended clock routing.
Figure 3. An example of an IF filter
amplitude response with respect to ADC
sampling rate that avoids frequency folding
The reference clock is usually generated
from an OCXO or a TCXO device, which
typically has very low jitter (or phase
noise). If the PLL reference clock’s frequency is chosen to be reasonably higher
Figure 4 summarizes the clock distribution scheme discussed above assuming a
reference frequency of 100MHz. The clock
buffer and distributor of Figure 4 has a
very important role in this system as it
receives a single-ended sine wave from the
OCXO or TCXO device, and delivers two differential LVPECL signals suitable for routing
to the ADC and the PLL. It should do so
IF SELECTIVITY FILTER
AMPLITUDE RESPONSE
The LO signal shown in Figure 1 is typically generated from a phase-locked loop
(PLL) system. The PLL requires a reference
clock to lock the LO to. Traditionally,
10MHz had been a common reference
frequency. However, much higher frequency reference clocks are becoming
more widespread nowadays. As a matter
of fact, 100MHz and higher frequencies
are not uncommon in modern RF designs.
than twice the received RF channel’s
bandwidth (or that of multiple channels
in a receiver where two or more adjacent
channels are digitized simultaneously),
the same reference signal can also be used
to clock the IF sampling ADC, following
some proper frequency planning. Ideally,
the IF selectivity filter’s passband and the
majority of its transition zone should fit
into a single Nyquist zone of the ADC to
avoid frequency folding. This point is
clarified with the help of the IF filter amplitude response given in Figure 3, where
the IF is chosen to match the 7th Nyquist
zone of the ADC. In Figure 3, fS stands for
the sampling rate of the ADC. In this case,
the LO in Figure 1 would be chosen such
that the down-converted signal output
of the mixer is centered in the middle of
the IF selectivity filter shown in Figure 3.
1st fS/2 2nd fS
NYQUIST NYQUIST
ZONE
ZONE
3fS/2
2fS
5fS/2
3fS
7th 7fS/2
NYQUIST
ZONE
FREQUENCY
July 2013 : LT Journal of Analog Innovation | 27
The LTC2153-14 is a 310Msps, 14-bit ADC that is
well-specified for high analog input frequencies, making
it suitable as the IF sampling ADC in this application.
CIRCUIT IMPLEMENTATION
CLOCK BUFFER AND DISTRIBUTER
Figure 4. The reference
clock distribution scheme
100MHz IF SAMPLING CLOCK, LVPECL
100MHz
REF IN
SINEWAVE
PLL
LO
100MHz PLL REF, LVPECL
while adding a minimum amount of jitter
to the distributed clock. The LTC6957-1 is
a low additive jitter dual LVPECL output
clock buffer that suits this application and
meets all the requirements set forth in the
discussion above. Other output formats
can be achieved by employing different
Figure 5. Schematic for the IF
sampling system employing two
Linear Technology demo boards
with input 315.5MHz test tone and
a 100MHz reference clock
versions of the LTC6957. The LTC6957-2
has LVDS outputs and the LTC6957-3
and LTC6957-4 offer CMOS outputs.
0.01µF
315.5MHz
T1
5.1Ω
BPF
As already discussed, jitter is one of the
main limiting factors for increasing the
IF. To find out what type of a performance can be achieved using a common
ADC along with the LTC6957-1 as the
clock distributor, two Linear Technology
demonstration circuits are modified
and hooked up as shown in Figure 5.
The LTC2153-14 is a 310Msps, 14-bit
ADC that is well-specified for high analog
input frequencies, making it suitable as
the IF sampling ADC in this application.
Its demonstration circuit, the DC1565A-G,
is modified as shown in Figure 5.
LTC2153-14
5.1Ω
AIN+
DA13
54Ω
6dB PAD
0.01µF
•
•
•
100Ω
5.1Ω
54Ω
5.1Ω
50Ω
T1: MABA-007159-000000
0.01µF
0.1µF
AIN–
•
•
•
DA0
VCM
ENC+
2.2µF
ENC–
100Ω
MODIFIED DC1565A-G DEMO BOARD
ADDITIVE JITTER = 145fS(RMS)
3.3V
0.1µF
0.01µF
LTC6957-1
FILTA
100MHz
0.01µF
10dBm
SINEWAVE
3dB PAD
50Ω
0.01µF
50Ω
DC1765A-A DEMO BOARD
28 | July 2013 : LT Journal of Analog Innovation
SD1
+
OUT1+
IN+
OUT1–
IN–
OUT2–
GND
OUT2+
V
FILTB
0.01µF
SD2
±PECL1OUT, 100MHz IF SAMPLING CLOCK
±PECL2OUT, 100MHz PLL REF
130Ω
130Ω
130Ω
130Ω
14-BIT OUTPUT
TO PC & PScope
design ideas
Calculation of the LTC6957-1 Additive Jitter
The demonstration circuit, DC1765A-A,
featuring the LTC6957-1, is used to buffer the sine wave output of a 100MHz
OCXO. One of the DC1765A-A differential LVPECL output pairs is connected to
the differential encode clock input of
the DC1565A-G. The second pair could
be used as the reference input of the
LO generating PLL shown in Figure 1.
Given the ADC is clocked at 100MHz, the
highest theoretical bandwidth that can
be achieved, while avoiding aliasing, is
50MHz. As shown in Figure 3, the 7th
Nyquist zone is picked, meaning that this
50MHz ideal bandwidth covers the 300MHz
to 350MHz frequency range. This would
require an ideal brick-wall bandpass filter
centered at 325MHz with a passband of
50MHz to pass only the IF information
present in the 300MHz to 350MHz range
while rejecting everything else that could
alias and interfere with the desired band.
Due to non infinitesimal transition
zone between the filter passband and
band-reject regions in a practical filter,
besides center frequency tolerance, a
more reasonable IF bandwidth selection in this case would be, for instance,
a surface acoustic wave (SAW) filter with
up to 30MHz bandwidth centered around
325MHz. SAW filters in this frequency range
are becoming more readily available.
PERFORMANCE SUMMARY
A 315.5MHz test tone is connected to the
analog input of the modified DC1565A-G
through a BPF that resembles the IF selectivity filter and an attenuator to dial the
amplitude seen by the ADC to –1dBFS.
In a sampling system, jitter is usually measured through a 2-step process. The first step is to take a baseline
SNR measurement with a relatively low frequency analog input tone at –1dBFS where jitter is not a major
contributor of noise. Call this measurement SNR_BASE. A second measurement is taken using the same sampling
clock source as in the first reading but with a higher-frequency analog input tone, still at –1dBFS. The SNR
should degrade with the second measurement if the input frequency is high enough to realize jitter-related SNR
degradation. Call this second measurement SNR_DEGRADED. It should be noted that in the second measurement,
jitter can have multiple sources, including the sampling clock, the ADC aperture jitter and the analog input signal.
Taking the RMS difference of the two measurements results in the jitter-limited SNR achieved at the higher input
frequency had the ADC had no quantization or thermal noise at its analog input. Call this calculated number
SNR_JTTR. These three terms are related as such:
 − 1 SNR _ DEGRADED)
− 1 SNR _ BASE) 

SNR _ JTTR = − 10 log 10 10 ( 10
− 10 ( 10
The achieved SNR (SNR_JTTR) due to tJ amount of total jitter at the encode input of the ADC for an analog input
tone at a frequency fIN is:
SNR _ JTTR = − 20 log 10 (2 π fIN t J )
Combining the last two equations and solving for tJ results in an equation that calculates the system jitter directly
from the two measurement outcomes mentioned above.
tJ =
1log
10 2


10 10
(
)
(
)
− 1 SNR _ DEGRADED
− 1 SNR _ BASE 
10

−10 10
2πfIN
The LTC6957-1’s jitter contribution is measured following the procedure outlined above. Two sets of
measurements are taken based on the schematic shown in Figure 5. The first measures the total intrinsic system
jitter, which includes the ADC’s aperture jitter and these for the 100MHz and 315.5MHz sources but excluding the
LTC6957-1. The second includes the LTC6957-1’s noise contribution. Taking the RMS difference between the two
measurements results in the LTC6957-1’s additive jitter.
The total intrinsic system jitter excluding the contribution from the LTC6957-1 is found by connecting the 100MHz,
13dBm source straight into the encode input of the ADC with the use of a transformer to drive the clock input
differentially. Two SNR measurements, excluding harmonics, are taken: one with a 10MHz, –1 dBFS sine wave at
the analog input of the ADC, which reads 67.8dB. The second SNR measurement is taken with a 315.5MHz, –1
dBFS tone at the analog input of the ADC, resulting in 65.3 dB of SNR. The formula derived above calculates the
total intrinsic system jitter:
TOTAL INTRINSIC SYSTEM JITTER =
1log
10 2


10 10
( )
( )
− 65 .3
− 67 .8 
10 −10 10 
2 π • 315 . 5M
= 181fS (RMS )
The total system jitter after adding the LTC6957-1 to the system as shown in Figure 5 is found by taking another
similar set of two measurements, one with an analog input of 10MHz and another with an analog input of
315.5MHz as described in the previous paragraph. The two SNR numbers are 67.8 dB and 64.24 dB, respectively.
Using the same jitter formula as above results in the total system jitter:
TOTAL SYSTEM JITTER =
1log
10 2


10 10
(
)
( )
− 64 .24
− 67 .8 
10 −10 1 0 
2 π • 315 . 5M
= 232 fS (RMS )
Taking the RMS difference between the intrinsic and total system jitter numbers gives the additive jitter
contribution of the LTC6957-1:
LTC6957 – 1 ADDITIVE JITTER = 232 2 − 1812 = 145 fS (RMS )
July 2013 : LT Journal of Analog Innovation | 29
The low jitter clock buffer and distributor, LTC6957-1, is employed to distribute
the 100MHz system reference clock in LVPECL format to be used as the ADC
sampling clock and the PLL reference. Performance of the IF sampling system is
measured by looking at the SNR and SFDR numbers. An excellent 64dB of SNR
and outstanding 80dB of SFDR are achieved with this system, enabling the relatively
high IF sampling which helps relax the RF image rejection filter requirements.
The DC1565A-G is connected via USB to a
PC, where PScope1 data acquisition control
software is used to look at two crucial
parameters that affect the quality of the
receiver: SNR and the SFDR. Figure 6 shows
PScope™ in action, displaying a 131072point FFT along with some analysis while
having the 315.5MHz, –1dBFS tone as the
analog input of the ADC and the 100MHz
LVPECL signal buffered by the LTC6957-1
as the ADC encode clock. As can be seen in
Figure 6, the achieved SNR is over 64dB and
the SFDR is over 80dB. These are excellent numbers for a 325MHz IF sampler.
Figure 6. Screenshot of PScope
showing the FFT and achieved
signal integrity parameters of the
system shown in Figure 5
30 | July 2013 : LT Journal of Analog Innovation
Because the input of the LTC6957-1 is
a 100MHz sine wave at +10dBm power
into 50Ω, its internal bandwidth limiting filters (FILTA and FILTB), which help
reduce the amount of added jitter when
the input is low in amplitude and/
or frequency, are both turned off per
LTC6957 data sheet recommendation.
CONCLUSION
A 325MHz IF sampling system, as part
of an RF receiver, is built and evaluated. The low jitter clock buffer and
distributor, LTC6957-1, is employed
to distribute the 100MHz system
reference clock in LVPECL format to be
used as the ADC sampling clock and
the PLL reference. Performance of the
IF sampling system is measured by looking at the SNR and SFDR numbers. An
excellent 64dB of SNR and outstanding 80dB of SFDR are achieved with
this system, enabling the relatively
high IF sampling which helps relax the
RF image rejection filter requirements. n
Notes
1PScope collects and analyzes data from the ADC in
both time and frequency domains, and displays relevant
parameters (available for download at www.linear.com).
design ideas
Near Noiseless ADC Drivers for Imaging
Derek Redmayne
CCDs (charge coupled devices) and other sensors place heavy demands on digitizers,
both in terms of sample rates, and in signal-to-noise ratio. The sensor output is typically
a ground-referenced series of analog levels (pixels), possibly with transients occurring
between the pixel boundaries. As the number of pixels increases, so does the sample
rate of the ADC required to capture the image, with 20Msps pipelined ADCs sufficient for
most high dynamic range applications. To ensure the highest SNR performance of the
sampled signal, the drive circuitry for the ADC must provide low impedance, fast settling
without introducing wideband noise and yet present high input impedance to the sensor.
This article presents an interface circuit
between the sensor and a high performance ADC that does not compromise
the SNR performance. The LTC2270
16-bit pipelined ADC family is intended
for high end imaging applications. The
84.1dB SNR of this family makes it attractive for imaging, but it also features
very good SFDR—over 100dB. The input
range is 2.1VP–P, significantly less than
the output of most imaging devices, so
attenuation and level-shifting is required.
The inputs of these ADCs must be driven
with a well-balanced differential drive. The
single ended drive normally available from
the sensor would force the internal virtual
grounds to absorb common mode input
current, which would cause degraded
performance. These ADCs are also very
low powered devices at 80mW/channel.
Differential drive is actually fundamental
to the low power operation since single
ended drive would require additional
power to maintain a stable internal
reference point in the ADC. These devices
operate on 1.8V supplies, imposing an
input range of this order, if nothing else,
simply to stay away from the rails. It is
important to stay away from the rails to
prevent differential phase error that would
be associated with the voltage variable
capacitance of internal protection diodes.
Therein lies a dilemma: A differential
amplifier capable of performing single
ended to differential translation without
compromising the SNR of the ADC will
necessarily have low input impedance (or
low value resistors) and as it must settle
quickly to 16-bit accuracy, will likely consume something on the order of four times
the power of the ADC itself. The LTC6409
differential amplifier is an example that
produces good results, but consumes
260mW, requiring that the surrounding network dissipate about 40mW just
to produce a level shift. Furthermore, it
dissipates signal power in the relatively
low value resistors required for low noise
and for maintaining phase margin.
These differential amplifiers as a result
have low input impedance. The buffer that would be required to present a
high impedance to the CCD also presents
a dilemma. It must be low noise, be
capable of settling in less than 25nsec,
and be capable of slewing with enough
dV/dt to maintain closed loop operation
during transients. Additionally, it must
be capable of driving the low input
impedance of the differential amplifier. Yet, the application demands low
power. This dilemma is even greater if
there is the expectation that the amplifier operate from the single supply rail.
Most differential amplifiers present a
number of problems as they either need
band-limiting after the amplifier to the
extent that settling is compromised, or
they are not very stable with a gain of
less than one (a noise gain of less than
two) and as such tend to ring. Many are
not common mode compatible with the
1.8V ADCs or they do not have enough
headroom to accommodate a double
terminated filter, or a level shift after the
amplifier. The LTC6404 is a good case to
study. It is stable at unity gain, does not
ring as do some, and could potentially be
used with an attenuator after the amplifier, but the input referred noise is 1.5nV.
This compares to 1.1nV for the LTC6409.
The LTC6404 noise density peaks considerably above 100MHz, it consumes 175mW,
and it is not really compatible with the
900mV common mode required by the
LTC2270. If it were followed by a filter
July 2013 : LT Journal of Analog Innovation | 31
The LTC2270 16-bit pipelined ADC family is intended for high end imaging applications.
The 84.1dB SNR of this family makes it attractive for imaging, but it also features
very good SFDR—over 100dB. The input range is 2.1VP–P, significantly less than the
output of most imaging devices, so attenuation and level-shifting is required.
R16
100Ω
R23
120Ω
R3
BUFF+ 402Ω
–
CCD
R1
75Ω
R21
50Ω
+
L7
220nH
+
U2
LT1395
R24
1.5k
–
R19
604Ω
BUFF–
C6
22pF
L3
22nH
R14
8.2Ω
L1
47nH
L5
27nH
C4
3.3pF
R13
22Ω
C9
3.3pF
L2
47nH
L6
27nH
C3
3.3pF
R18
8.2Ω
C8
3.3pF
R22
22Ω
L4
22nH
R26
100Ω
BUFF+
PROVIDED BY
IMAGING
SUPPLY BOARD
CMA+
V5
0.9V–1.1V
R27
100Ω
R11
50Ω
R7
820Ω
R4
453Ω
R17
332Ω
R6
820Ω
R5
453Ω
R15
332Ω
+
+
–
+
U1
LT1395
–
CFAOUT
LTC2270
AIN+
R9
25Ω
C1
17pF
C5
1.8pF
C2
1.8pF
R10
25Ω
AIN–
R12
50Ω
CMA+
–
T1
tD = 300ps
Z0 = 50Ω
C7
17pF
C10
1.8pF
T2
tD = 300ps
Z0 = 50Ω
COMMON MODE SERVO
CMA–
R25
1.5k
+
–
V2
8V
+
–
V4
5V
+
–
V1
2V
+
–
V3
5V
BUFF–
R20
604Ω
R2
100Ω
+
–
V6
LTC6655
2.25V
CMA–
and level shift, the impedance would
be such that you will dissipate some
80mW dropping voltage in the dividers.
You could potentially operate the amplifier off +3.3V and –2V to resolve common
mode compatibility and produce a larger
signal swing without requiring a level
shift after the amplifier. But the negative
supply is not often palatable to designers.
The settling time available to the amplifier may not be an entire clock cycle.
32 | July 2013 : LT Journal of Analog Innovation
Figure 1. Near noiseless circuit for interfacing a CCD to a high performance ADC
The source may dictate this, but there is
a disturbance produced by the ADC on
the opposite edge of the clock and this
would give the filter only ½-clock cycle
to settle even if the amplifier were not
otherwise disturbed. If the amplifier is
disturbed by this event, it does not leave
as much time for the filter to settle.
A simple RC would require about 14 time
constants to settle to 16 bits, and for
20Msps, this would result in a bandwidth
of about 90MHz. As it happens, the amplifier will be disturbed to some extent
by sampling and this means that the
bandwidth of a simple post filter must
be extended to 130MHz to 150MHz to
allow for some settling of the amplifier in
response to the disturbance. Unfortunately,
this will pass noise in the region where
the amplifier is peaking. A higher order
filter may result in a more pronounced
reduction of noise contribution from
design ideas
If an ADC with high SNR is required in an imaging application, a singleended to differential conversion is required to get the signal from the CCD
to the ADC. The conversion must attenuate the signal swing and provide a
very stable common mode output level without adding significant noise.
Figure 2. Prototype imaging board
Figure 3. Prototype 0.5 square
inch 4-output power supply
the earlier Nyquist zones, but will
not necessarily settle very quickly.
The scheme described herein can
drive the 25Msps LTC2270 family with
84.1dB SNR and 17pF sample capacitors.
Impedances can be raised and power
consumption can be reduced at sample
rates of 20Msps, and less. For sample
rates higher than 30Msps, a more conventional topology involving a fast buffer,
followed by a differential amplifier like
the LTC6409 is required. The LTC6404-1
can alternatively be used in that case.
THE NEAR NOISELESS SUGGESTION
The circuit in Figure 1 shows a suggested
drive scheme that results in almost no loss
of SNR with the LTC2270 family, yet can
settle to 16 bits within 1 pixel at 25Msps.
The noise at –84.0dB (all included) is such
that it will not necessarily be within 1
count in a single frame, but averaging
multiple frames could resolve to 16 bits.
The buffer amplifier (U2) is a current
feedback amplifier used essentially as
an emitter follower. The bulk of the
output current is taken via R16, seemingly from the emitters, although
power is delivered from the output.
As the impedance looking into the inverting input is low compared to the feedback
network, the output noise is attenuated
at the inverting input and, as a result, the
inverting input noise current does not
contribute significantly. This amplifier has
a voltage noise specification of 4.5nV/√Hz,
although when used as a unity gain buffer, inverting input noise current in the
minimum value feedback resistor produces
10.1nV/√Hz. However in this emitterfollower-like mode of operation, it appears
to be on the order of 1.5nV/√Hz to 2nV/√Hz.
There is a feedback loop around the
amplifier and this amplifier imposes a
minimum feedback resistor of 400Ω.
However, at low frequency, the feedback
impedance is 400Ω in parallel with R23
to reduce the excursion required at the
output. But at high frequency, the feedback is the required minimum 400Ω.
There is a minor amount of output
power taken from the output, via R24,
just to reduce the required excursion
produced at the output. But this may not
be required in many cases, for example
where the video signal is 0V–4V, or less.
There is provision for R24 just in case, in
the future, U2 may be a different amplifier
where it is practical to take power from
the output. There are several possible
alternatives depending on required slewing. A low noise fast settling FET amplifier
could possibly be used on a single supply. With rail-to-rail amplifiers, it is likely
that the positive rail must be 6V to avoid
crossing through the transition region
between two input stages, a region that
causes distortion. If the LT1395 is used,
7.5V to 8V must be used for VCC and –2V for
VSS, if intended to receive 0V–5V signals.
A lower powered amplifier such as the
LT6252 may be used if there is no need to
settle through a full-scale step between
pixels. Bad pixels however would bleed
into following pixels. The presence of
clock feed-through and the actual settling
time available may limit these choices.
July 2013 : LT Journal of Analog Innovation | 33
Figure 4. Two tone test
–7.022dBfs sinusoid
at 70kHz, –7.01dB
synchronous square wave
at Nyquist
The second amplifier is also an LT1395, but
note that this cannot be a dual LTC1396,
unless the application were to involve
signals centered on ground potential.
This second stage must operate on 5V and
–5V in order to sink current and perform
a level shift from ~2.5V common mode to
0.9V common mode, as well as provide
differential drive by virtue of controlling the common mode. The noise and
distortion contribution of this amplifier
is largely rejected by the CMRR of the
ADC as its influence is only common mode,
assuming the network surrounding the
amplifier is completely symmetrical.
We developed a power supply board
that provides all four of the required
voltages from a single 5V input. The
power supply circuit is capable of powering four channels, yet produces no
evidence of the 1.2MHz switching rate
used by the LT3471—even at –125dBFS.
As shown, this driver produces
84.0dB SNR combined with the ADC and
34 | July 2013 : LT Journal of Analog Innovation
including any contribution from the power
supply board. The tests below were done
with R1 at 75Ω, and a 50Ω source. This
circuit should be suitable where CCDs
either have an output impedance in the
50Ω–200Ω range, or where the charge
transfer into a holding cap produces an
effective impedance in the hundreds of
ohms. The use of a fast FET buffer may
allow very high source impedance.
The capacitor C6, at 22pF, is required
if the dV/dt during transients from the
CCD exceeds the slew rate of the LTC1395
or if RFI is present. CCDs appear to tolerate capacitive loads of this magnitude.
If the output stage of the LTC1395 is not
capable of keeping up, conduction of input
protection diodes would greatly reduce the
input impedance. This conduction occurs
in almost any feedback amplifier. And
this would produce an error if a charge
transfer mechanism were exposed to this
input current, possibly even if buffered
in the CCD. R21 is potentially desirable as
source termination if the distance between
C6 and the amplifier were extended.
This topology is only practical with ADCs
that have approximately 2VP–P input range
and where the CCD signal is on the order
of 0V–4V or 0V–5V. This takes advantage
of the attenuation that is required to
produce the balun action by controlling
the common mode. This is analogous to
the transmission line balun where high
common mode impedance between input
and output ports results in balanced drive
if symmetrically terminated to AC ground.
The filter as shown produces a Gaussianlike response, and is 3dB down at about
40MHz. The filter is replicated twice
independently, in order to provide a symmetrical network that maintains the error
contribution of U1 as common mode.
R7, R4, and R17 and their counterparts
satisfy the requirements for stability of U1,
produce attenuation of the 0V–5V signal
to ±1V, and produce a level shift. These
design ideas
Figure 5. Same applied
power level at 70kHz,
but removal of power at
Nyquist
elements can in fact be after the ADC and
act as end termination, which allows
somewhat faster settling. If there is a
significant distance between the CCD and
the ADC, simulation says that the transmission path can be extended between
a pair of 50Ω resistors replacing R16. If
the distance is between 30cm and up to
about 60cm, the cable should be 75Ω. The
source termination resistor would then
be 75Ω and the other side, 25Ω. PCB traces
should be higher than 75Ω if possible.
If this is intended to drive into the
LTC2185, for example, a 350psec transmission path is possible. If the LTC2270
family is used, the 17pF sample capacitors require that this transmission
line (T1 and T2 in Figure 1) should
be less than 40psec (about 1cm).
Tests performed to satisfy that this will
perform with CCD signals, besides digitizing small offset frequencies from ¼FS and
½FS at 20Msps and 25Msps, include: 300kHz
–1dBFS sinusoid (–92dB SFDR 2nd and 3rd),
representative of dV/dt in a CCD signal;
as well as a near full scale square wave
(10MHz and 5MHz) with a superimposed
–20dB sinusoid at 200kHz, showing
no distortion on the sinusoid resulting
from the large synchronous excursion
in pairs of “black and white pixels.”
Note the appearance of two waveforms in
the time domain plot in Figure 4, caused
by alternation between the two levels in
the square wave every other sample.
The inverse FFT window is zoomed in
on the time axis, showing only the tone
at Nyquist, this by selectively masking out the power in the 70kHz area.
Figure 5 shows no apparent power
change in the low frequency tone, still
at –7.022dBFS and only relatively minor
change in the distortion components.
This proves the addition of a high
amplitude square wave is not causing
compression at the peaks. A 70kHz tone,
superimposed on a ½FS square wave
is believed to represent the dV/dt and
settling scenario that would be representative of that seen in a CCD waveform
sampled near the end of the pixel.
CONCLUSION
If an ADC with high SNR is required in
an imaging application, a single-ended
to differential conversion is required to
get the signal from the CCD to the ADC.
The conversion must attenuate the signal
swing and provide a very stable common mode output level without adding
significant noise. The circuit presented
here can do just that. Working with a
low power ADC that has a data sheet
SNR specification of 84.1dB, this circuit
achieved 84.0dB—implying that the
conversion was nearly noiseless. n
July 2013 : LT Journal of Analog Innovation | 35
Low Power, DC Accurate Drivers for 18-Bit ADCs
Guy Hoover
A common misconception about 18-bit SAR ADCs is the only way to drive them
is with a high powered, high speed, low noise op amp or differential driver whose
DC performance often leaves much to be desired. It is possible to use a low
power, DC accurate op amp to drive an 18-bit SAR ADC if the input is a DC or low
bandwidth AC signal and the op amp outputs are given sufficient time to settle.
Using the single-ended to differential driver of Figure 1, four different low power
dual op amps were tested to show the trade-offs in using low power drivers.
This circuit enables the testing of offset voltage, the maximum sample
rate for DC and AC signals, noise and
power consumption. The ADC used
is the LTC2379-18, a 1.6Msps 18-bit
SAR ADC with an offset of ±9LSBs, INL of
±2LSBs and peak-to-peak noise of 5LSBs.
The four amplifiers used are the
LT1013, LTC6078, LTC6081 precision and
LTC2051HV zero-drift dual op amps—
Table 1 shows a summary of their major
specs. The circuit supply voltage is the
voltage applied to the V+ and V– terminals
of the op amps. This voltage determines
the power dissipation of the amplifier and
its maximum signal swing. The voltages
were chosen to maximize undistorted
signal swing without exceeding the maximum ratings of the amplifiers or unnecessarily increasing power dissipation.
2.5V
10µF
0.1µF
0.1µF
LTC6081
VIN
0V TO 5V
VCM
2.5V
+
–
4.99k
340Ω
10k
10k
–
+
660pF
NP0
660pF
NP0
340Ω
660pF
NP0
VDD
OVDD
IN+
LTC2379-18
IN–
REF
GND
REF
CHAIN
RDL/SDI
SDO
SCK
BUSY
CNV
REF/DGC
10µF
SAMPLE CLOCK
VREF
5V
47µF
Figure 1. Low power, DC accurate single-ended
to differential driver for the LTC2379-18
OFFSET VOLTAGE
The worst-case offset of the LTC2379-18
is only ±340µV. As Table 1 shows, the
offset voltage of these four op amps is
even lower, varying from ±3µV max for
the LTC2051HV to ±150µV max for the
LT1013. To maintain this low level of
offset voltage careful circuit design and
layout practices are required. Examples
of this include using the 4.99kΩ resistor
Table 1. Op amp comparison
PART
I SY (µA) TYP/AMP
V OS (µV) MAX
CIRCUIT SUPPLY VOLTAGE(V)
LT1013A
350
±150
8, –3
LTC2051HV
1000
±3
8, –3
LTC6078
55
±30
5.9, 0
LTC6081
340
±70
5.9, 0
36 | July 2013 : LT Journal of Analog Innovation
3.3V
5.9V
in the positive input of the inverting
amplifier to balance the input bias current created voltage drops and using
symmetrical layout around the positive
and negative inputs of the ADC, minimizing the effects of the parasitic elements.
MAXIMUM SAMPLING RATE
FOR DC SIGNALS
Even with a DC input voltage there is
some settling time required. When the
ADC goes from hold mode to sample
mode, the sample capacitor is switched
onto the analog input pin of the
ADC producing a brief transient. The
sample capacitor is then charged by
the op amp back to its final value.
The maximum sampling frequency for
a DC input voltage can be determined
design ideas
The worst-case offset of the LTC2379-18 is only ±340µV. The offset
voltage of the four op amps is even lower, varying from ±3µV max for the
LTC2051HV to ±150µV max for the LT1013. To maintain this low level of
offset voltage careful circuit design and layout practices are required.
Figure 2. Schottky
diodes flatten
falling edge of
input pulse
1N5711
1N5817
Figure 3. Data acquisition setup
OP AMP
INPUT
50Ω
by reducing the sampling frequency
until there is no ripple on the sampled
waveform. Take this measurement
with the analog input near positive
or negative full scale as this typically
causes the largest transients to be generated at the analog inputs, which in
turn require the maximum settling time.
Maximum sampling rates for the four
op amps vary from 63ksps to 230ksps.
The maximum sample rates for the
four op amps are shown in Table 2.
MAXIMUM SAMPLING RATE
FOR AC SIGNALS
For applications with a slow moving
analog signal or a multiplexer at the
driver input, determining the maximum
sampling rate is more complex. With the
potential for a full-scale swing from one
measurement to the next it is necessary
to consider the settling of the RC filter
at the output of the driver circuit and
the settling time of the op amp itself to
an 18-bit level. Because settling to the
18-bit level is almost never specified, it
must be experimentally determined.
–9V
GND
+9V
TRIGGER
CLK
OP AMP
DRIVER
BOARD
+
DEMONSTRATION
CIRCUIT
DC1783A-E
DEMONSTRATION
CIRCUIT
DC718
−
CH1 OF
81110A
It is important to keep the stray capacitance on the op amp input as small
as possible to minimize the settling
time of this circuit. By swapping the
outputs of the SE-Diff driver circuit of
Figure 1 it is possible to look at both
falling and rising edge settling times.
With the flat input edge in place, one can
look at the ADC output to see how long
the output of the SE-Diff driver takes to
settle. One method involves using the
PScope data acquisition software and
carefully choosing the sample rate and
input frequency. Using a 50kHz sample
rate to ensure that all four op amps
settle completely once the input signal
is constant would normally only allow
a 20µs resolution in the settling time
Table 2. DC input maximum sampling rate
Drive the op amps with a flat falling
edge as shown in Figure 4 so that the
settling of the op amps is measured;
not the slow change of the DC input.
The schematic for the edge shaping circuit is shown in Figure 2.
STROBE OUT
OF 81110A
PART
DC INPUT MAXIMUM SAMPLE
RATE (ksps)
LT1013A
185
LTC2051HV
230
LTC6078
63
LTC6081
165
USB TO PC
measurement. By using the primitive
wave capability of PScope and picking
the sampling frequency, input frequency
and sample size such that multiple passes
of the input signal are sampled at different time slices it is possible to reassemble the samples to create a high
resolution image of the input signal.
The formula used to determine the ratio
of input frequency to sample frequency is
M
• fS = fIN
N
where N is the sample size. N must be 2i
where i is any integer from 10 to 17 for
PScope. M is any odd number from 1 to
N/2. fS is the sample frequency and fIN is
the input frequency. Picking a sample
size of 131072 to maximize resolution,
an input frequency of 250Hz and M = 653
to get a sample rate of approximately
50ksps yields an fS of 50.18070444ksps.
The full resolution shown is necessary,
as the primitive wave requires an exactly
coherent relationship between sample
rate and input frequency. The clock
and the input signal generator must be
synchronized. Figure 3 shows the test
July 2013 : LT Journal of Analog Innovation | 37
Figure 4. PScope primitive wave shows
LT1013 settling time for falling edge
with 30.5ns/sample resolution
–125140
RAW
AVG16
AVG64
–125145
CODE
–125150
–125155
–125160
–125165
0
100
200
TIME (µs)
300
400
Figure 5. LT1013 settling
setup. The strobe signal makes sure that
PScope starts capturing at the same point
for each series of samples and requires
that Start on Trigger is enabled in the
Tools menu. To calculate the required
clock frequency, multiply the desired
sample frequency by 62 when using
the DC1783A-E. This results in an input
clock frequency of 3.111203675MHz.
of this measurement, it is possible to take
the raw data from PScope, export it to
Excel or MATLAB, and then reassemble the
primitive wave and average the resulting
data. The raw results as well as the averages of 16 readings and 64 readings for the
four op amps are shown in Figures 5–8.
The settling times and peak-to-peak noise
are summarized in Table 3. Note that the
LTC2051HV, which had the fastest sample
rate for a DC signal has the slowest settling
time for a full scale input swing. This is a
result of the time required to auto-zero the
difference in offset voltage caused by the
change in common mode input voltage.
The PScope output for the LT1013 is
shown in Figure 4. With 131072 samples
divided into the 4ms period of the input
waveform, a resolution of 30.5ns/point is
achieved. By zooming in on the start and
stop points of the primitive waveform it
is possible to calculate an approximate
settling time. This is limited by the peakto-peak noise. To increase the accuracy
Removing the Schottky diodes and
driv­ing a 20Hz sine wave into VIN of the
SE-Diff input with a sampling frequency of
Table 3. Settling times and peak-to-peak noise
approximately 1/(settling time) yields the
THD numbers for the circuit of Figure 1
shown in Table 4. Good THD numbers are
an indication of the linearity of the circuit.
NOISE
As shown in the summary of Table 3, the
combination of the SE-Diff driver and the
LTC2379-18 yields a peak-to-peak noise
that is 1.5 to 3 times higher than the
ADC by itself. With relatively modest averaging it is possible to obtain noise levels
that approach 1LSB peak-to-peak. As long
as the noise is Gaussian and not caused by
clock feedthrough or some other synchronous source, averaging should reduce the
noise by the square root of the number
of the samples. Averaging reduces the
Table 4. THD
PART
SETTLING
TIME (µs)
PK-PK NOISE
(LSBs)
AVERAGE OF 16 SAMPLES
PK-PK NOISE (LSBs)
AVERAGE OF 64 SAMPLES
PK-PK NOISE (LSBs)
PART
f S (kHz)
THD (dB), A IN
= –1dBFS
LT1013
80
10
2.2
1.1
LT1013
12.0
–105
LTC2051HV
3000
14
2.9
1.2
LTC2051HV
0.3
–104
LTC6078
140
12
3.3
1.7
LTC6078
7.0
–98
LTC6081
130
8
1.8
0.9
LTC6081
7.0
–105
38 | July 2013 : LT Journal of Analog Innovation
design ideas
–130365
–130625
RAW
AVG16
AVG64
–130370
–130725
RAW
AVG16
AVG64
–130630
RAW
AVG16
AVG64
–130730
–130735
–130385
CODE
–130635
–130380
CODE
CODE
–130375
–130640
–130740
–130645
–130745
–130390
–130395
–130400
0
2
1
3
–130650
50
100
TIME (ms)
Figure 6. LTC2051HV settling
effective sampling rate and may not be
practical with non-repetitive AC signals.
POWER CONSUMPTION
Table 5 shows the power dissipation of
the op amps and the LTC2379-18 at the
maximum DC and AC sampling frequencies of the single-ended to differential
data acquisition circuit of Figure 1. The
op amp power dissipation is per amplifier and does not change with sampling
frequency because the op amps are always
on. The LTC2379-18 power dissipation is
linear with sampling frequency due to its
auto shutdown after a conversion. The
combined circuit power dissipation in
columns four and six of Table 5 shows
the dissipation of the two op amps and
the LTC2379-18 used in the circuit added
150
200 250
TIME (µs)
300
350
400
–130750
Figure 7. LTC6078 settling
together. At the higher sampling rates
that are possible with DC input signals,
the power dissipation of the LTC2379-18
is a significant portion of the total. At the
lower sampling rates required for AC or
muxed input signals, the op amps dissipate
almost all of the power for this circuit.
CONCLUSION
Using low power, DC-accurate op amps to
drive 18-bit SAR ADCs is possible as long as
enough time is allowed for the op amps
to settle. Settling time is the limiting factor in determining the circuit’s maximum
sampling rate, and varies greatly by op
amp choice and the type of signal source,
DC or AC. ADC offset and linearity can be
maintained using low power op amps.
Due to the higher noise of the low power
50
100
150
200 250
TIME (µs)
300
350
400
Figure 8. LTC6081 settling
op amps, some averaging may be required
to reduce the noise at the expense of a
lower effective sampling rate. At lower
sample rates power dissipation is limited
by the op amp driver circuitry, which
must remain on so that it can settle, while
the ADC can typically be powered down
after only a brief conversion period. It
is important to pick an op amp that is a
good match to the offset, sampling rate,
noise, linearity and power requirements
of your overall circuit requirements. n
Table 5. Power dissipation at maximum sampling frequencies for AC and DC input signals
PART
OP AMP P D (mW) AT
CIRCUIT SUPPLY VOLTAGE
LTC2379-18 P D (mW)
AT MAX DC f S
COMBINED CIRCUIT P D
(mW) AT MAX DC f S
LTC2379-18 P D
(mW) AT MAX AC f S
COMBINED CIRCUIT P D
(mW) AT MAX AC f S
LT1013
3.85
2.08
9.78
0.135
7.835
LTC2051HV
11.00
2.59
24.59
0.003
22.003
LTC6078
0.32
0.71
1.35
0.080
0.720
LTC6081
2.01
1.86
5.88
0.080
4.100
July 2013 : LT Journal of Analog Innovation | 39
highlights from circuits.linear.com
70V
7
WIDE COMMON MODE RANGE 10X GAIN
INSTRUMENTATION AMPLIFIER
The LTC6090 is a high voltage precision operational
amplifier. The low noise, low bias current input stage
is ideal for high gain configurations. The LTC6090 has
low input offset voltage, a rail-to-rail output stage, and
can be run from a single 140V or split ±70V supplies..
circuits.linear.com/619
3
+IN
2
+
5
8
LTC6090
–
4
9
6
22pF
10k*
1
100k
22pF
1
LT5400-2
100k
2
7
8
3
7
100k
–70V
205k
70V
7
3
–IN
70V
2
+
4
3
100k
5
9
22pF
8
6
2
6
5
4
LTC6090
–
24.9k
100k
5
8
LTC6090
–
4
100k
9
49.9Ω
6
OUT
–3dB at 45kHz
1
22pF
9
10k*
+
1
–70V
–70V
RWIRE
0.5Ω
IN
VIN
RIN
200Ω
LT3080
+IN V+
ISET
RS
VREG
IOUT
LT6110
WIRE LOSS COMPENSATION USING A CURRENT REFERENCED LDO
This circuit shows a cable drop compensation circuit using a current
referenced regulator, the LT3080. A precision 10μA set current, ISET, is
sourced through two series connected resistors to program the output
voltage for the remote load. To compensate for the load connecting
cable drop requires sourcing an additional current into this resistor pair
to increase the output voltage. The LT6110 provides a sourced current
at the IMON pin which is also directly proportional to the current flowing
to the load. This current is three times the normal IOUT current.
circuits.linear.com/637
–IN
LOAD
20mΩ
+
–
VLOAD
3V
1A
+ –
SET
RSET1
301k
RSET2
1.69k
IMON
IMON
V–
IMON = 3 IIN+
DUAL SUPERCAPACITOR BACKUP POWER SUPPLY, 0.5V TO 5V
The LTC3122 is a synchronous step-up DC/DC converter with
true output disconnect and inrush current limiting. The 2.5A
current limit along with the ability to program output voltages up
to 15V makes the LTC3122 well suited for a variety of demanding
applications. Once started, operation will continue with inputs
down to 500mV, extending run time in many applications.
circuits.linear.com/638
L1
3.3µH
VIN
0.5V TO 5V
CIN
4.7µF
OFF ON
SW
SD
LTC3122
PWM/SYNC
SC1
50F
SC2
50F
VIN
C1
100nF
R1
383k
CAP
COUT
100µF
FB
RT
VCC
RT
57.6k
VOUT
5V
VOUT
VC
SGND
CVCC
4.7µF
PGND
RC
43.2k
CC
1nF
R2
121k
CF
68pF
CIN, CVCC: 4.7µF, 6.3V, X7R, 1206
C1: 100nF, 6.3V, X7R, 1206
COUT: 100µF, 6.3V, X7R, 1812
L1: TDK SPM6530T-3R3M
SC1, SC2: MAXWELL BCAP0050-P270
L, LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode, LTspice, TimerBlox and µModule are registered trademarks, and Hot Swap, PowerPath, PScope and UltraFast are trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
© 2013 Linear Technology Corporation/Printed in U.S.A./61.3K
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