LT Journal of Analog Innovation V26N2 - May 2016

May 2016
I N
T H I S
I S S U E
multi-output clock
synthesizer with integrated
VCO and low jitter 12
negative current-reference
linear regulator 20
Volume 26 Number 2
20A LED Driver with Accurate ±3%
Full Scale Current Sensing Adapts to
Multitude of Applications
Josh Caldwell and Walker Bai
load sharing for three or
four supplies with unequal
voltages 26
monolithic SEPIC/boost
regulators with wide VIN
range, high efficiency,
and power-on reset and
watchdog timers 28
Rapidly evolving LED lighting applications are replacing nearly
all traditional forms of illumination. As this transformation
accelerates, power requirements for LED drivers increase,
with higher currents making it more challenging to maintain
current sensing accuracy without sacrificing efficiency. LED
drivers must do this while managing current delivery to
multiple independent LED loads at high speeds, and be able
to connect parallel drivers with accurate current sharing.
Some high power LEDs have unique mechanical and
electrical considerations, where the anode is electrically
tied to the thermally conducting backtab. In a traditional
LED driver with a step-down regulator configuration,
where thermal management is achieved by cooling the
chassis, the anode connection to the backtab creates a
mechanical-electrical design challenge. The backtab must
have good thermal conductivity to the heat sink, but
also be electrically isolated from it if the voltage at the
tab is different from the chassis. Since is it difficult for
LED manufacturers to change processing or packaging,
the LED driver itself must meet this design challenge.
The LTC4125 5W AutoResonant wireless power transmitter features foreign object
detection and completes linear wireless charging solutions (see page 31).
w w w. li n ea r.com
One option is to use a 4-switch positive buck-boost LED
driver, but the additional switching MOSFETs add system
complexity and cost. An inverting buck-boost topology
uses only one set of switching power MOSFETs, and allows
the anode heat sink to be tied directly­—electrically and
(continued on page 4)
Linear in the News
In this issue...
COVER STORY
SINGAPORE TEST FACILITY EXPANDS TO BOOST PRODUCTION
CAPACITY
20A LED Driver with Accurate ±3% Full Scale
Current Sensing Adapts to Multitude of Applications
In February, Linear opened the company’s third semiconductor test facility in
Singapore. The additional space for staff, equipment and materials will allow
the company to more than double its production capacity of analog circuits and
µModule® (power module) products. This strengthens Linear’s ability to meet the
growing demand for high performance analog integrated circuits worldwide.
Josh Caldwell and Walker Bai
1
DESIGN FEATURES
Multi-Output Clock Synthesizer with Integrated
VCO Features the Low Jitter Required to Drive
Modern High Speed ADC and DAC Clock Inputs
Chris Pearson
12
1.5A, Negative Regulator Expands Family of
Current-Reference Linear Regulators
Dawson Huang
20
DESIGN IDEAS
The new 87,000 square foot facility is located beside Linear Technology’s
current 191,000 square foot facility in Singapore. The company’s Singapore
test operation, started 27 years ago in 1989, has sophisticated capabilities for
high volume testing of the company’s numerous products, including many
package types, tape and reel, as well as pack and ship to customers worldwide.
The location also includes Linear’s Singapore Design Center and the area sales
office supporting Singapore, Malaysia, India and Australia/New Zealand.
Continued Expansion
What’s New with LTspice IV?
Gabino Alonso
24
Easy Balanced Load Sharing for Three or Four
Supplies, Even with Unequal Supply Voltages
Vladimir Ostrerov and Chris Umminger
26
Design Once; Use Twice: Monolithic SEPIC/Boost
Regulators with Wide VIN Range Satisfy Requirements
of Both Consumer and Commercial Vehicles
Molly Zhu
28
new product briefs
30
back page circuits
32
Over the years, Linear has continued to expand its Singapore test operations, with expansion of its first building in 1997 and a second five-story
building completed in 2005. This third major expansion, adding an additional 87,000 square foot test facility, is now complete. With head count of
nearly a thousand employees today, Linear has a highly experienced team,
capable of testing the most high performance analog ICs. The facility provides
test capability for over 90 percent of the company’s global demand.
Over the past decade, Linear’s products have grown in sophistication
and complexity. This has paralleled the company’s increasing participation in the key electronics growth markets of industrial and automotive.
Industrial, at 44 percent of the company’s business in the most recent fiscal
year, is a broad market that includes industrial process control, factory
automation, robotics, instrumentation and medical, among others.
Linear has also enjoyed significant growth in its automotive business, reaching
20 percent of the company’s business. This market is driven by the increasing use of electronic systems, replacing mechanical and other systems, in
all aspects of the car, from safety and navigation to electronic steering and
braking and many systems under the hood. This technical transformation
has resulted in analog and power devices that are increasingly complex, and
require sophisticated test systems, now installed in the company’s Singapore
facility. These test systems enable Linear to provide the high quality and
reliability demanded by industrial and automotive manufacturers.
2 | May 2016 : LT Journal of Analog Innovation
Linear in the news
Linear’s Singapore
test facility expands
to boost production
capacity.
receive the highest quality products
in the shortest possible lead times.
CONFERENCES & EVENTS
Sensor+Test 2016, Nuremberg Exhibition Centre,
Nuremberg, Germany, May 10–12, Booth 241,
Hall 1—Showcasing Linear’s broad line
of high performance ICs, including
SmartMesh IP™ solutions. There will
be working demo applications from
European customers using Linear’s
products. www.sensor-test.de/welcome-tothe-measurement-fair-sensor-test-2016/
Internet of Things World 2016, Santa Clara
In addition to test capability for the
company’s analog and power ICs, the
Singapore facility has extensive test capability for Linear’s growing line of µModule
power products. These are complete power
systems-in-package with integrated inductor, MOSFET, DC/DC regulator ICs and
supporting components. Each µModule
product is thoroughly tested using Linear’s
stringent electrical, package and thermal
reliability tests. The new Singapore
expansion provides the company with
additional capacity for testing µModule
products to meet growing demand.
components of our global manufacturing
operations. Given the excellent conditions
for growth in the region, as well as the
skill and dedication of our employees here,
Singapore will remain as the headquarters
and focal point for all Linear Technology
operations in Asia. We will continue to
enhance our capabilities here to deliver
advanced technology solutions to rapidly
growing markets around the world.”
Also located in Singapore is the
company’s reliability testing center.
This is a fundamental part of Linear’s
operations, focusing on ensuring
superior product quality for automotive and other demanding markets.
Lothar Maier, Chief Executive Officer
of Linear Technology, added, “I have
high confidence in our Singapore test
infrastructure and the talent base we’ve
cultivated here over many years, and
we expect to grow it. Our advanced test
operations here ensure that our customers
in the demanding and fast growing automotive and industrial markets continue
to receive the highest quality products.”
Grand Opening
Complete Manufacturing Capability
In February, Linear’s management team
was in Singapore for the opening of
the expanded test facility. They toured
the new facility and spent time with
the Singapore operations team.
Linear’s Singapore test operation is just
one part of the company’s advanced
manufacturing operations. These
include two wafer manufacturing
facilities, located in Milpitas, California
and Camas, Washington, as well as
a wafer sort and package assembly
operation in Penang, Malaysia. These
facilities ensure that Linear customers
Robert Swanson, Linear Technology
co-founder and Executive Chairman,
who flew to Singapore for the opening,
said, “The facilities in Singapore are vital
Convention Center, Santa Clara, California,
May 10–12—Linear’s Dust Networks®
wireless sensor network products.
https://iotworldevent.com/
Space Tech Expo, Pasadena Convention Center,
Pasadena, California, May 24-26, Booth 8025—
Showcasing products for space and harsh
environments. www.spacetechexpo.com/
NXP FTF Technology Forum, JW Marriott, Austin,
Texas, May 16–19—Power management
ICs
and µModule products. www.nxp.com/
support/classroom-training-events/nxp-ftftech-forum:NXP-FTF-TECH-FORUM-HOME
Wireless Japan 2016, Tokyo Big Sight, Japan,
May 25–27, Booth 1233—Demonstrating
Linear’s Dust Networks wireless
sensor network products and their
ecosystem in Japan. www.wjexpo.com
Sensors Expo & Conference, McEnery Convention
Center, San Jose, California, June 21–23, Booth
940—Presenting Linear’s energy harvest-
ing family and low power wireless sensor
networks. www.sensorsexpo.com/
IEEE Nuclear and Space Radiation Effects
Conference, DoubleTree and Oregon Convention
Center, Portland, Oregon, July 11–15, Booth
33—Showcasing products for space and
harsh environments. www.nsrec.com/ n
May 2016 : LT Journal of Analog Innovation | 3
To meet high performance demands, the LT3744 can be configured
as a synchronous step-down or inverting buck-boost controller
to drive LED loads at continuous currents exceeding 20A. The
supply input for the LT3744 is designed to handle 3.3V to 36V.
(LT3744, continued from page 1)
Full-range analog current regulation
accuracy is 3%, and even at 1/20th scale,
it is better than ±30%. The LT3744 has
three independent analog and digital
control inputs with three compensation
and gate drive outputs for a wide range
of LED configurations. By separating
the inductor current sense from the LED
current sense, the LT3744 can be configured as a buck or inverting buck-boost.
For ease of system design, all input
signals are referenced to board ground
(SGND, signal ground), eliminating the
need for complex discrete level-shifters.
In the inverting buck-boost configuration, the total LED forward voltage
can be higher than the input supply
voltage, allowing high voltage LED
strings to be driven from low voltage
supplies. When PCB power density calls
for spreading the component power
4 | May 2016 : LT Journal of Analog Innovation
100
380 TYPICAL UNITS
VCTRL1 = 0V
90
80
NUMBER OF UNITS
To meet high performance demands, the
LT®3744 can be configured as a synchronous step-down or inverting buck-boost
controller to drive LED loads at continuous currents exceeding 20A. The supply
input for the LT3744 is designed to handle
3.3V to 36V. As a step-down converter,
it regulates LED current from 0V up
to the supply voltage. As an inverting
buck-boost converter, the LT3744 can
accurately regulate LED currents with
output voltages from 0V down to −20V.
125°C
25°C
–45°C
250
NUMBER OF UNITS
mechanically—to the chassis ground,
eliminating the need for electrical isolators on the heat sink, and simplifying
the mechanical design of the system.
300
200
150
100
125°C
25°C
–45°C
380 TYPICAL UNITS
VCTRL1 = 1.5V
70
60
50
40
30
20
50
10
0
–300 –200 –100
0
100
200
300
REGULATED VLED_ISP - VLED_ISN VOLTAGE (µV)
0
59
59.4
59.8
60.2
60.6
61
REGULATED VLED_ISP - VLED_ISN VOLTAGE (mV)
Figure 1. The LED current regulation amplifier
in the LT3744 has a typical offset of ±300µV with
VCTRL = 0V.
Figure 2. At full current, the LED current regulation
loop has a typical accuracy of ±1.7% with VCTRL =
1.5V.
dissipation, the LT3744 can be easily
paralleled with other LT3744s to drive
high pulsed or DC currents in LED loads.
to 1/20th of the total current control
range. This is critical in applications
where the total digital PWM dimming
range is limited—or in applications where
very high dimming range is required.
As an example, with a 100Hz PWM
dimming frequency and a 1MHz switching frequency, the LT3744 is capable of
1250:1 PWM dimming, which can be
combined with 20:1 analog dimming to
extend the total diming range to 25,000:1.
HIGH ACCURACY CURRENT
SENSING
The LT3744 features a high accuracy
current regulation error amplifier, which
achieves accurate analog dimming down
PWM1
5V/DIV
SW
10V/DIV
ILED
1.67A/DIV
IL
20A/DIV
1µs/DIV, 5-MINUTE PERSISTENCE
Figure 3. The LT3744 features flicker-free LED
dimming.
Figure 1 shows the production consistency
of the LT3744 with regard to offset voltage
over temperature, in this case 380 typical
units when the analog control input is
at 0V. With the low offset of the error
amplifier, the control loop is capable of
a typical accuracy of ±10% at 1/20th
scale analog dimming. The distribution
of the regulated voltage across the LED
current sensing pins with the control
input equal to 1.5V is shown in Figure 2.
The accuracy at full range is better than
design features
In projection systems, reducing the turn-on time of the light source reduces timing
constraints. With a reduction in timing constraints, the image refresh rate can
increase, allowing higher resolution images and a reduction in the rainbow effect
from fast-moving white objects. The LT3744 is capable of transitioning between
the different output current states in less than three switching cycles.
Figure 4. The LT3744 is capable of driving
a single LED with three different current
levels.
EN/UVLO
VIN
EN/UVLO
PWM1
PWM2
PWM3
CTRL1
CTRL2
CTRL3
1µF
TG
220nF
BOOST
SW
LT3744
2V
VREF
2.2µF
RHOT
45.3k
L1
1.2µH
INTVCC
RNTC
680k
BG
CTRLT
VEE
ISP
ISN
SGND
RS
3mΩ
M2
SS
RT
82.5k
VEE
VC1
287k
C2
330µF
M4
D2
D3
PWM_OUT2
VFNEG
PWM_OUT3
M3
20A MAXIMUM
C3
330µF
M6
D1
SYNC PWM_OUT1
10nF
L1: IHLP-5050FD-ER1R2M01
RS: WSL28163L000D
RSLED: WSL28163L000J
M1: BSC050NE2LS
M2: SiR438DP
M3, M4, M5, M6, M7, M8: Si7234DP
D1, D2. D3, D4: BAT54A
C1, C2, C3: 10T4B330M
C1
330µF
22µF
100k
FAULT
M1
VIN
56µF 24V
×4
BLUE
51k
M8
M5
10µF
M7
D4
LED_ISP
LED_ISN
FB
VC2
VC3
287k
10nF
RSLED
3mΩ
287k
10nF
10k
10nF
±3%, which corresponds to ±1.8mV on
the 60mV full-scale regulation voltage.
FLICKER-FREE PERFORMANCE
One of the most important metrics in LED
driver performance is in the recovery of
the LED current during PWM dimming.
The quality of the end product is highly
dependent on the behavior of the driver
in the first few switching cycles after the
rising edge of the PWM turn-on signal.
The LT3744 uses proprietary PWM,
compensation and clock synchronization
technology to provide flicker-free performance—even when driving LEDs to 20A.
Figure 3 shows a 5-minute capture of the
LED current recovery with a 12V supply
delivering 20A to a red LED. The switching frequency is 550kHz , the inductor
is 1µ H, the PWM dimming frequency
is 100Hz with an on-time of 10µsec
(1000:1 PWM dimming). Roughly 30,000
dimming cycles are shown, with no
jitter in the switching waveform—every
recovery switching cycle is identical.
HIGH SPEED DIMMING BETWEEN
THREE DIFFERENT REGULATED
CURRENTS
In projection systems, reducing the
turn-on time of the light source reduces
timing constraints. With a reduction in
timing constraints, the image refresh rate
can increase, allowing higher resolution
images and a reduction in the rainbow
effect from fast-moving white objects.
The LT3744 is capable of transitioning
between the different output current
states in less than three switching cycles.
The LT3744 features three regulated
current states, allowing color-mixing
system designers to sculpt the color
temperature of each LED. Color mixing
delivers high color accuracy, corrects
inaccurate LED colors, and eliminates
variations in production systems. While
the LT3743 has low and high current states,
the LT3744 features three current states so
that all three RGB LED colors can be mixed
with each other at their own light outputs
to independently correct the other colors.
Figure 4 shows a 24V input/20A output,
single LED driver with three different regulated currents—determined by the analog
voltages on the CTRL and the digital
state of the PWM pins. Note that since
RS is only used for peak inductor current
and absolute overcurrent protection,
May 2016 : LT Journal of Analog Innovation | 5
Within miniature “pocket” or smartphone projection systems, total solution space
and cost are paramount. The LT3744 combines switched output capacitor
technology with a floating gate driver to create a complete RGB solution from
a single LED driver, a significant space savings over multi-IC drivers.
A COMPLETE RGB LED SOLUTION
FOR POCKET OR SMARTPHONE
PROJECTORS
it does not need to be a high accuracy
resistor—which reduces system cost.
PWM dimming between the three differ-
Within miniature “pocket” or smartphone
projection systems, total solution space
and cost are paramount. In these applications, PCB space is extremely limited and
the total volume of the driver solution
(including component height) must be
minimized. Using only one LED driver
for all three LEDs drastically reduces
space—allowing use of larger batteries or higher power LEDs for improved
battery lifetime and projected lumens.
ent current states is shown in Figures
5 and 6. In Figure 5, the PWM signals
are sequentially turned on and off.
PWM3 has the highest priority and
PWM1 has the lowest. This allows
rapid, single input signal transitions to
change the output current. As shown
in Figure 6, there can be any arbitrary
interval between the PWM input signals.
Figure 7. The LT3744 is capable of driving
all three color component (R, G and B)
LEDs in a pocket or smartphone projector
from a single Li-ion battery.
EN/UVLO
EN/UVLO
PWM1
PWM2
PWM3
CTRL1
CTRL2
CTRL3
VIN
2V
VREF
RHOT
45.3k
L1
6.8µH
FAULT
CTRLT
VEE
ISP
ISN
20A MAXIMUM
330µF
M2
D1
VEE
VC1
B
G
M5
D4
VFNEG
M8
107k
M7
LED_ISP
LED_ISN
FB
VC2
VC3
RSLED
12mΩ
10k
6.8nF
6.8nF
6 | May 2016 : LT Journal of Analog Innovation
G
M6
D3
PWM_OUT2
6.8nF
D5
330µF
M3
D2
PWM_OUT3
40.2k
2.2µF
330µF
M4
10nF
RT
RS
6mΩ
22µF
SYNC PWM_OUT1
SS
Each LED can be turned on sequentially,
with a time delay in between, or with any
L1: MSS1048-682NL
RS: WSPL08056L000FEA18
RSLED: WSLP1206R0120D
M1: BSC010NE2LSI
M2: SiR438DP
M3, M4, M5, M6, M7, M8: Si7234DP
D1, D2. D3, D4: BAT54A
D5: PMEG4010
M1
220nF
INTVCC
BG
SGND
VIN
3.3V
VEE
TG
100k
RNTC
680k
47µF
20µF
BOOST
SW
LT3744
2.2µF
The LT3744 combines switched output
capacitor technology with a floating gate
driver to create a complete RGB solution
from a single LED driver. The LT3744
uses a unique gate driver for the PWM
output pins. The negative rail of the driver
floats on the VFNEG pin, allowing it to
pull down the gates of all of the switches
that are off to negative voltages. This
ensures that the switches in-series with the
output capacitors do not turn on in any
condition. This driver allows up to a 15V
difference between any string of LEDs.
VEE
R
design features
In addition, with the three independent analog control inputs, each
LED can operate at a different regulated current. When the LT3744 is
configured as an inverting buck-boost, a single lithium-ion battery can
drive three independent LED strings using only a single controller.
Summary of Linear’s high power LED driver-controller family
LT3741
LT3743
LT3744
LT3763
LT3791
V IN range
6V–36V
6V–36V
3.3V–36V
6V–60V
4.7V–60V
LED output range
0V–34V
0V–34V
−20V–36V
0V–55V
0V–52V
Topology
buck
buck
buck and inverting
buck-boost
buck
buck-boost
LED current regulation accuracy
±6%
±6%
±3%
±6%
±6%
⁄ 10 scale LED current accuracy
±60%
±60%
±17%
±60%
±35%
50mV
50mV
60mV
50mV
100mV
1
Full-scale LED current sense
Common anode connection for LEDs
L
LED fault indication
L
L
L
Low side LED PWM gate driver(s)
0
2
3
1
1
Individual LED current states
1
2
3
1
1
pattern input into the PWM digital inputs.
In addition, with the three independent
analog control inputs, each LED can
operate at a different regulated current.
When the LT3744 is configured as an
inverting buck-boost, a single lithiumion battery can drive three independent
LED strings using only a single controller. Figure 7 shows a 3.3V/5A inverting
tri-color buck-boost LED driver designed
specifically for RGB pocket projectors.
PWM1
5V/DIV
PWM1
5V/DIV
PWM2
5V/DIV
PWM3
5V/DIV
PWM2
5V/DIV
PWM3
5V/DIV
ILED
6.67A/DIV
ILED
6.67A/DIV
25µsec/DIV
Figure 5. The LT3744 transitions between any of
three regulated current states and off in less than
three switching cycles.
25µsec/DIV
Figure 6. The different current states can be turned
on at any time—with or without time in between
each state.
324W 2-LED DRIVER USING TWO
PARALLEL LT3744 LED DRIVERS
A significant limiting factor in any high
power/high current controller design is
power density in the PCB. PCB power
density is limited to roughly 50W⁄cm2
to prevent excessive temperature rise
within the power path components.
In extreme cases, where an LED load
requires more power than a single driver
can support (while remaining within
power density limits), multiple converters can be paralleled to spread the load.
An efficient high current LED drivercontroller, with modern power MOSFETs,
can provide roughly 200W (at a solution
size of approximately 4cm2) and limit all
power path component temperatures to
under 80ºC. For LED loads higher than
200W, the LT3744 can be paralleled with
other LT3744s to limit the temperature rise
May 2016 : LT Journal of Analog Innovation | 7
100k
PWM1
EN/UVLO
PWM1
PWM2
PWM2
22µF
1µF
D1
VIN
FAULT
U1
LT3744
CTRLT
100k
M2
470µF
2.43k
ISP
D5
D6
FB
SYNC
PWM_OUT1
RT
PWM_OUT2
SS
VFNEG
10nF
604Ω
1nF
LED_ISP
SGND VEE VC1
VC2
LED_ISN
M5
M9
M6
M10
M13
M15
1nF
226k
2mΩ
ISN
CTRL2
100k
0.22µF
BG
CTRL1
100k
2.2µF
L1
0.82µH
470µF
100k
100k
47µF
×2
M1
SW
VREF
1nF
VIN
12V
BOOST
TG
82.5k
56µF
×2
INTVCC
PWM3
100k
10µF
×4
25.5k
1nF
226k
25.5k
10nF
10nF
D3
2mΩ
VIN
100k
1µF
22µF
EN/UVLO
PWM1
VIN
BOOST
PWM3
TG
82.5k
1nF
FAULT
VREF
U2
LT3744
BG
D7
PWM_OUT2
VFNEG
10nF
1nF
LED_ISP
SGND VEE VC1
226k
1nF
470µF
2.43k
D8
PWM_OUT1
SS
226k
M4
FB
SYNC
RT
1nF
2mΩ
ISN
CTRL2
100k
0.22µF
ISP
CTRL1
100k
2.2µF
47µF
×2
L2
0.82µH
470µF
CTRLT
100k
M3
D1, D2: NXP PMEG4002EB
D3–D8: BAT54A
L1, L2: VISHAY IHLP-5050FD-ERR82
M1, M3: BSC032NE2LS
M2, M4: BSC010NE2LS
M5–M12: Si7234DP
M13–M16: BSC010NE2LS
SW
100k
100k
56µF
×2
INTVCC
PWM2
100k
10µF
×4
D2
VC2
25.5k
10nF
LED_ISN
604Ω
M7
M11
M8
M12
M14
25.5k
10nF
M16
D4
2mΩ
Figure 8. A 57A/324W 2-LED driver
8 | May 2016 : LT Journal of Analog Innovation
design features
Figure 11. Parallel board
temperatures at 100% duty
cycle delivering 324W to the
LED
INDUCTOR
SWITCHING
MOSFETS
CHANNEL 1
CHANNEL 2
9A/DIV
LT3744
10ms/DIV
VIN = 12V
VOUT = 4V
IOUT = 54A
fSW = 400kHz
100% DUTY CYCLE
Figure 9. LED current sharing during start-up
controllers in this design produces 27A—
for a total of 54A at 6V. By tying the
corresponding compensation outputs
together, both controllers behave in
unison to provide a smooth, well behaved
start-up and accurate DC regulation.
CHANNEL 1
9A/DIV
CHANNEL 2
9A/DIV
20µs/DIV
Figure 10. DC LED current sharing at full load—
showing very little variation between the two parallel
drivers
in any particular component. All compensation outputs should be paralleled, allowing current sharing between each regulator.
Figure 8 shows a 324W converter using
two Linear DC2339A demo boards
connected in parallel. Each of the parallel
Figure 9 shows the LED current start-up
behavior of each board. Note that the
regulated current provided by each board
is identical throughout the entire startup sequence. In DC regulation, without
PWM dimming, Figure 10 shows excellent current sharing between the two
application boards (the waveforms are
directly on top of each other). Figure 11
shows that the temperature rise above
ambient of the board at 100% duty
cycle is about 55ºC. Component L1 is the
Figure 13. Parallel board
temperatures at 50% PWM
dimming delivering 54A
pulses to the LED
PWM
2V/DIV
inductor, Q1 and Q3 are the switching
power FETs, R5 is the inductor current
sense resistor, R32 is the LED current
sense resistor, and U1 is the LT3744.
In this application, two independent LED
strings can be PWM dimmed at the full
54A. When PWM dimming, Figure 12
shows that the LED current is completely
shared between the two drivers. In this
test, the rise time of the current in the LED
from 0A to 54A is 6.6µs. The electrical
connection from the output of each driver
to the LED must be carefully balanced to
avoid added inductance in either path—
which reduces the effective rise time.
Figure 13 shows the temperature
rise in each demo board with a 50%
PWM-dimmed LED current of 54A. To
INDUCTOR
SWITCHING
MOSFETS
CHANNEL 1
CHANNEL 2
9A/DIV
LT3744
20µs/DIV
Figure 12. The LT3744 features excellent LED current
sharing between parallel drivers during PWM
dimming.
VIN = 12V
VOUT = 4V
IOUT = 54A
fSW = 400kHz
50% DUTY CYCLE
May 2016 : LT Journal of Analog Innovation | 9
10µF
56µF
22µF
1µF
100k
VIN
EN/UVLO
PWM1
PWM1
BOOST
PWM3
100k
SYNC
TG
SYNC
82.5k
1nF
VEE
INTVCC
PWM2
100k
10µF
×2
D1
VEE
FAULT
VREF
U1
LT3744
CTRLT
L1
1.3µH
0.22µF
1mΩ
10µF
M2
BG
470µF
ISN
CTRL2
VEE
4.02k
VEE
ISP
CTRL1
100k
M1
SW
100k
100k
Figure 14. This parallel inverting
application delivers 120W to a
chassis tied common-anode LED.
VIN
12V
D5
FB
2.2µF
PWM_OUT1
1nF
PWM_OUT2
1nF
226k
226k
1nF
VFNEG
1k
M5
LED_ISP
SGND VEE RT
VEE
VC1 LED_ISN
SS
M6
D3
33nF
143k
10nF
VEE
VEE
3mΩ
VEE
VEE
VIN
10µF
56µF
1µF
100k
22µF
VEE
EN/UVLO
PWM1
100k
100k
VIN
1nF
PWM2
TG
FAULT
M3
L2
1.3µH
0.22µF
1mΩ
SW
VREF
U2
LT3744
CTRLT
BG
ISP
CTRL1
100k
D1, D2: NXP PMEG4002EB
D3–D6: BAT54A
L1, L2: WÜRTH 7443551130
M1, M3: BSC026N04LS
M2, M4: BSC018N04LS
M5–M8: Si7234DP
BOOST
PWM3
100k
100k
VEE
INTVCC
SYNC
82.5k
10µF
×2
D2
10µF
M4
470µF
ISN
CTRL2
VEE
4.02k
VEE
D6
FB
2.2µF
PWM_OUT1
1nF
226k
PWM_OUT2
1nF
1nF
VFNEG
226k
M7
LED_ISP
SGND VEE RT
SS
143k
VEE
VEE
VC1 LED_ISN
M8
D4
33nF
10nF
1k
VEE
VEE
3mΩ
VEE
10 | May 2016 : LT Journal of Analog Innovation
design features
By regulating the LED current directly and level-shifting all input signals, the
LT3744 is capable of producing negative voltages, allowing low voltage battery
operated systems to drive multi-LED strings with a simple 2-switch solution.
minimize the inductance from each of the
demo boards to the LED, the parallel LED
driver boards were mounted directly on
top of each other. A more optimized layout
would feature both drivers mounted on
a single board, with the driver layouts
mirroring each other, reflected across their
mutual connection to the LED. Whenever
designing the conduction path from a
LED driver to a high current LED, careful
attention should be placed on the total
inductance. Since inductance is a function of wire length, the longer the wire,
the longer the current recovery in the
LED—no matter how fast the driver.
INVERTING BUCK-BOOST, 120W
LED DRIVER WITH TWO PARALLEL
LT3744s
Inverting buck-boost applications have the
same thermal concerns as non-inverting
converters, with the additional design
challenge of increased inductor current.
For low input voltages and high LED voltages, the average current in the inductor
could be very high. For example, if the
input is 3.3V and the output is one green
LED—which has a forward voltage of 6V
at 20A—the peak inductor current is 70A.
The inductor used in the design should
have a saturation current at least 20%
higher—in this case, greater than 80A.
Since this current flows in the switching
MOSFETs, the MOSFETs must be rated
for greater than 80A. By placing two
LT3744 inverting buck-boost converters
in parallel, the peak switched current is
cut in half, reducing the requirements
of the power path components.
Figure 15. Parallel inverting
board temperatures
delivering 120W to the LED
INDUCTOR
SWITCHING
MOSFETS
LT3744
VIN = 12V
VOUT = −4V
IOUT = 30A
fSW = 350kHz
In the inverting buck-boost topology, the
inductor current is delivered to the load
only during the synchronous FET conduction time. If the two parallel converters
are allowed to run at their free-running
frequencies, there is noticeable beat
frequency apparent in the LED current
ripple resulting from the slight switching
frequency differences. To avoid this, each
converter uses the same RT resistor value,
but they are synchronized using an external clock. In the application in Figure 14,
the converters are designed to run at a
non-synchronized frequency of 300kHz —
with a 350kHz synchronizing clock.
Figure 15 shows the component temperature rise when delivering 30A to the LED in
a parallel inverting buck-boost application.
CONCLUSION
With features including high current
regulation accuracy, a floating PWM gate
driver, and level shifted input signals, the
LT3744 can be configured to drive LEDs
in a wide range of applications. The
LT3744 has the capability to be used as the
single driver in an RGB projection system,
drastically reducing total solution space—
making it possible to produce high lumen
video projection from a smartphone.
Through the use of three current regulation states, the LT3744 gives system
designers freedom to sculpt LED color,
producing more faithful video images. By
regulating the LED current directly and
level-shifting all input signals, the LT3744
has the capability to produce negative
voltages, allowing low voltage battery
operated systems to drive multi-LED
strings with a simple 2-switch solution.
The LT3744 can be easily paralleled
with other LT3744s to efficiently deliver
extremely high current to an LED, while
maintaining current accuracy and sharing
even when PWM dimming. Paralleling the
LT3744 lowers board temperatures, reduces
inductor currents and expands supported
LED power to hundreds of watts. n
May 2016 : LT Journal of Analog Innovation | 11
Multi-Output Clock Synthesizer with Integrated VCO Features
the Low Jitter Required to Drive Modern High Speed ADC
and DAC Clock Inputs
Chris Pearson
The latest high performance ADCs cannot realize their
potential without an ultralow jitter high speed clock signal.
The LTC6951 satisfies the requirements of top ADCs by
producing a clock signal up to 2.7GHz with an impressively
low wideband noise floor. Figure 1 compares the LTC6951’s
measured ADC SNR results to other ADC clock sources.
The quantity and performance requirements for low jitter clocks in electronic
systems continues to increase with system
complexity and performance. This may
result in a costly array of parts, including
VCOs, PLLs, clock distribution devices and
synchronization components to support
the clock signals. The LTC®6951, on the
other hand, decreases complexity and cost
by integrating a high performance PLL/
VCO, and distributing five ultralow jitter
clock outputs. Additionally, the LTC6951
supports several software-based synchronization methods: EZSync™, ParallelSync™
and EZ204Sync™ (aka EZParallelSync™).
The latest trend in high speed converter
digital interfaces is the adoption of the
JESD204B standard. Previous generation
clocking devices are often incompatible
with the JESD204B standard due to different synchronization and output divider
requirements. The LTC6951 accounts for
these differences, making it capable of
supporting JESD204B subclass 1. The
LTC6951 introduces Linear’s unique
reference alignment synchronization method known as ParallelSync,
which allows parallel LTC6951s to
clock multiple JESD204B devices.
Figure 1. LTC6951 performance advantage
Figure 2. LTC6951 block diagram
MEASURED ADC SNR (dBFS)
85
RAO = 0
LTC6951
jitter = 115fsRMS
80
PFD
CP
R DIV
75
The PLL section works in conjunction with
the external reference and the internal
4GHz to 5.4GHz VCO to generate the
desired VCO frequency (fVCO) as follows:
for RAO = 0:
f •N
fVCO = REF
R (1)
for RAO = 1:
f • N • P • M0
fVCO = REF
R
(2)
LTC6951
REF
PFD
70
P DIV
65
D0 DELAY
M0 DIV
Competitors
60 jitter = 140fsRMS
jitter = 200fsRMS
D1 DELAY
M1 DIV
D2 DELAY
M2 DIV
D3 DELAY
M3 DIV
D4 DELAY
M4 DIV
LTC2107 AIN = –1dBFS
0
100 200 300 400 500 600 700 800 900
ADC INPUT FREQUENCY (MHz)
12 | May 2016 : LT Journal of Analog Innovation
CP
R DIV
N DIV
N DIV
VCO
55
Referring to Figure 2, the LTC6951 features
two different configurations based on the
setting of the RAO (Reference Aligned
Output) register bit. The desired synchronization method determines which configuration is selected. The LTC6951 is divided
into three main circuit blocks: the phaselocked loop (PLL) and voltage controlled
oscillator (VCO) section, the clock distribution section and the digital control section.
RAO = 1
LTC6951
REF
THE INNER WORKINGS OF THE
LTC6951
VCO
TUNE
P DIV
OUT0
OUT1
OUT2
OUT3
OUT4
M0 DIV
D1 DELAY
M1 DIV
D2 DELAY
M2 DIV
D3 DELAY
M3 DIV
D4 DELAY
M4 DIV
TUNE
OUT0
OUT1
OUT2
OUT3
OUT4
design features
The LTC6951 produces clock frequencies up to 2.7GHz with the lowest wideband noise
floor in the industry for a clock distribution device. This allows the LTC6951 to directly
clock high speed ADCs with very challenging SNR and clock-to-clock skew targets.
where fref is the reference input frequency,
R is the reference input divide value, N
is the PLL feedback divide value, P is
the prescalar divide value, and M0 is the
output divider value. When RA0 = 0, N
is the VCO feedback divide value. When
RA0 = 1, the LTC6951 is in reference
aligned output mode and N • P • M0 is
the VCO feedback divide value. Reference
aligned output mode allows the user
to align the outputs of one or multiple
LTC6951s to the reference input.
The clock distribution section receives a
signal at fVCO /P, where P is the P-divider
value. After the P-divider, the clock signal
is distributed to five separate channels.
When RAO = 0, each of the five channels
can independently delay the first synchronized clock edge by any integer from 0 to
255 P-divider clock cycles. When RAO = 1,
OUT0’s delay option is disabled. After the
delay function, each channel can independently divide the frequency from a
list of divider values that range from 1 to
512. The output signal from the dividers
is sent to a buffer that determines the
output signal type. Four channels produce
an ultralow noise differential CML clock
signal capable of output frequencies up
to 2.7GHz. The fifth channel creates a
differential LVDS output that can produce
clock frequencies up to 800MHz.
The third and final section is the digital
control section, which controls the
various synchronization functions and is
discussed in detail in the “Synchronization
Methods” section of this article. The
digital control section includes a standard
Summary of Linear’s clock generation and distribution devices
Internal PLL
LTC6950
LTC6951
L
L
Internal VCO
Output fMAX (MHz)
LTC6954
LTC6957
L
1400
2700
1800
300
Outputs
5
5
3
2
Max output divide ratio
63
2048
63
1
EZSync
L
L
L
ParallelSync
L
JESD204B Subclass 1 compatible
L
PC-based design, simulation and
demo board control
ClockWizard™
4-wire serial interface and a pin to
monitor the status of certain register bits.
PERFORMANCE
There is a trade-off between using filtered
ADC clocks for optimal SNR performance
or unfiltered ADC clocks for optimal
clock-to-clock skew performance. There
are several applications with challenging
clock jitter and clock-to-clock skew
requirements. Examples include JESD204B
converters, and multi-arrayed systems
such as medical scanners and smart
array antennas. Filtering multiple clocks
for performance while accounting for
variations in filter delays to meet skew
requirements can be problematic. The
LTC6951 addresses these design challenges by providing multiple CML clock
outputs with 115fsRMS jitter and ±20ps
clock skew. For larger arrayed clock
systems requiring multiple LTC6951s,
±100ps clock skew can be achieved.
LTC6951Wizard
LTC6954_GUI
To determine the jitter requirement for an
ideal ADC clock input, refer to Equations
3 and 4. Equation 3 calculates the total
clock jitter required to achieve a desired
SNR level at a known full scale analog
input frequency. Equation 4 determines
the ADC clock input jitter requirement
after removing the ADC aperture jitter
from the total clock jitter. The ADC
aperture jitter number is usually provided
in the ADC data sheet. Equation 3 and
Figure 1 highlight that as an ADC’s analog
input frequency increases, lower jitter
clocks are required to achieve optimal
SNR performance. For a more in-depth
discussion of clock jitter requirements for
ADCs refer to the LTC6951 data sheet.
–SNRdB
10 20
tJ(TOTAL) =
2 • π • fIN
(3)
tJ(CLK _IN) = tJ(TOTAL)2 − tJ(APERATURE)2
(4)
fIN = ADC analog input frequency
May 2016 : LT Journal of Analog Innovation | 13
SYNCHRONIZATION METHODS
Table 1. Synchronization selection table
Architecture
Jitter
Timing requirements
Phase alignment
(all outputs)
EZSync
Standalone
EZSync
Multichip
ParallelSync
EZ204Sync
(EZParallelSync)
Standalone
Clock
Distribution
Reference
Distribution
Reference Divide
and Distribution
Ultralow
Low
Ultralow
Ultralow
Easy
Easy
Moderate
Easy
Yes,
at Time 0*
Yes,
at Time 0*
Yes,
at Time 0*
Yes, phases aligned
per LTC6951 sync
*Time 0 alignment implies all outputs requiring synchronization are phase aligned on the same sync event.
Figure 3. EZSync Standalone
EZSync CONTROLLER
LTC6951
REF CLK
OUT3+
OUT3–
ALIGNS ALL
CLOCK EDGES
AT t0
OUT2+
OUT2–
SYNC
Figure 4. EZSync Multichip
EZSync CONTROLLER
LTC6951
REF CLK
REF
OUT4+
OUT4–
OUT3+
OUT3–
OUT0+
OUT0–
OUT1+
OUT1–
SYNC
≥1ms
OUT2+
OUT2–
the LTC6951’s five outputs after toggling
the LTC6951’s sync pin or the SPI register SSYNC bit. This method showcases
the best jitter, clock skew performance
and easiest synchronization method.
number of synchronized clock outputs
by using the LTC6951 as an EZSync
CONTROLLER. This method maintains the
simple EZSync synchronization timing
requirements. However, for FOLLOWER
devices (Figure 4), such as the LTC6950
and LTC6954, the clock jitter performance
becomes additive in nature, as shown
in Equation 5 and Figure 5. Clock skew
performance depends on several factors,
including board trace length differences
between EZSync devices, FOLLOWER
propagation delays, and individual EZSync
device skew performance. The EZSync
clock skew performance can be optimized
using the LTC6951 output delay SPI bits.
OUT1+
OUT1–
≥1ms
EZSync Standalone (Figure 3) synchronizes
EZSync Multichip (Figure 4) increases the
OUT4+
OUT4–
OUT0+
OUT0–
REF
The LTC6951 provides three synchronization methods: EZSync, ParallelSync
and EZ204Sync (or EZParallelSync).
The advantages and disadvantages
of each method are summarized in
Table 1 and in the descriptions below.
tJ(EZSyncTOTAL) =
EZSync FOLLOWER
tJ(EZSyncCNTRLLR)2 + tJ(EZSyncFLLWR)2
LTC6954-1
OUT0+
OUT0–
IN+
IN–
OUT1+
OUT1–
OUT2+
OUT2–
SYNC
–100
VCO–
LV/CM+
LV/CM–
PECL0+
PECL0–
PECL1+
PECL1–
PECL2+
PECL2–
SYNC
14 | May 2016 : LT Journal of Analog Innovation
PECL3+
PECL3–
–110
PHASE NOISE (dBc/Hz)
SYNC PULSE SKEW
BETWEEN ANY TWO
SYNC PINS
MUST BE < 10µs
VCO+
(5)
Figure 5. Phase comparison
ALIGNS ALL
CLOCK EDGES
AT t0
EZSync FOLLOWER
LTC6950
EZSync Multi-Chip
Follower Outputs
–120
–130
–140
–150
EZSync Standalone
ParallelSync
EZ204Sync (EZParallelSync)
–160
–170
1k
10k
100k
1M
OFFSET FREQUENCY (Hz)
10M 40M
design features
ParallelSync synchronization (Figure 6) increases
REFERENCE DISTRIBUTION
the number of synchronized clock outputs
by distributing the reference to multiple
LTC6951s. This method maintains the
LTC6951 jitter performance provided by
the EZSync Standalone method, since
the out of band reference input noise is
removed by the LTC6951 loop filter, as
shown in Figure 5. The synchronization
timing requirements are a function of
the reference frequency (refer to Figure 6
for SYNC to REF timing diagram). The
clock skew performance depends on
board trace length differences between
the reference distribution circuit and
the LTC6951, reference clock skew and
individual LTC6951 output skews. The
clock skew performance can be optimized
using the LTC6951 output delay SPI bits.
LTC6954-4
ParallelSync synchronization uses the
LTC6951’s Reference Aligned Output
mode (RAO = 1 in Figure 2), which
provides a known latency between the
falling edge of the sync input signal and
the starting edge of all LTC6951 outputs.
Figure 8’s ParallelSync timing diagram
explains how outputs from one or
multiple LTC6951’s can be programmed
to begin at a desired point in time.
EZ204Sync (or EZParallelSync) (Figure 7) is
a simple multichip synchronization
method targeted at, but not limited
to, JESD204B applications requiring
CLOCK and SYSREF signals. EZ204Sync
maintains the jitter performance of
ParallelSync, but with easier implementation. This is accomplished by using
an EZSync distribution device to act as
an external R-divider to the PLL/VCO
reference inputs, as shown in Figure 7.
The outputs of all PLL/VCOs are phase
aligned. However, this architecture allows
the phase alignment of the multiple
PLL/VCO devices to take place on any
R-divider cycle. As a result, phase alignment of each LTC6951 is performed
independently, enabling the user to
REF CLK
VCC
10k
10k
10k
IN
1µF
REF+
100Ω
OUT0+
OUT0–
REF–
OUT1+
OUT0SEL
1µF
LTC6951
OUT3+
RAO = 1 OUT3–
OUT1–
OUT1SEL
OUT0+
OUT0–
OUT2+
OUT2–
OUT2SEL
OUT4+
OUT4–
OUT1+
OUT1–
OUT2+
OUT2–
SYNC
1µF
REF+
100Ω
REF–
Figure 6. ParallelSync
1µF
LTC6951
OUT4+
OUT4–
ALIGNS ALL
CLOCK EDGES
AT t0
OUT3+
RAO = 1 OUT3–
OUT0+
OUT0–
CK
SYNC
PULSE
D
OUT1+
OUT1–
CK
Q
D
Q
OUT2+
OUT2–
SYNC
LTC6951
REF INPUT
tSS
LTC6951
SYNC PULSE
(SEE DATASHEET
FOR tSH AND tSS)
REFERENCE DISTRIBUTION
1µF
REF+
LTC6954-4
REF CLK
/2
/32
SYNC
PULSE
≥1ms
SYNC
OUT0+
100Ω
OUT0–
OUT1+
OUT1–
REF–
1µF
OUT2+
OUT2–
CS
SYNCHRONIZATION STEPS
1) EZSync—REFERENCE DISTRIBUTION
2) SPI SYNC—LT6951s
tSS
SYNC TO REF TIMING
SYNC HELD HIGH A MINIMUM OF 1ms
SPI
SYNC
SCLK
SDI
OUT4+
OUT4–
CLOCK
+
LTC6951 OUT3
OUT3–
RDIV = 1
RAO = 1 OUT0+
OUT0–
OUT1+
OUT1–
OUT2+
OUT2–
ALIGNS ALL
CLOCK EDGES
1µF
REF+
100Ω
REF–
Figure 7. EZ204Sync (EZParallelSync)
1µF
SPI
SYNC
power on and off individual LTC6951s
without requiring resynchronization of
all LTC6951s. This ability to synchronize
CS
SCLK
SDI
OUT4+
OUT4–
SYSREF
+
LTC6951 OUT3
OUT3–
RDIV = 1
RAO = 1 OUT0+
OUT0–
OUT1+
OUT1–
OUT2+
OUT2–
individual LTC6951s independently is ideal
for JESD204B subclass 1 applications.
May 2016 : LT Journal of Analog Innovation | 15
The LTC6951’s variety of synchronization methods
allows designers to optimize for ease of synchronization,
clock jitter and the number of clocks required.
JESD204B INTERFACE
Figure 8. ParallelSync timing diagram
JESD204 is a serial data converter digital
SYNC TO REF
INPUT
SETUP TIME
1 RDIV OUTPUT CYCLE
OUTx DELAY SETTING
(ADDITIONAL PDIV CYCLES)
18 PDIV
CYCLES
SYNC
REF
INPUT
RDIV
OUTPUT
(INTERNAL
SIGNAL)
OUT0
OUT1
OUT2
OUT3
OUT1 DELAY SETTING = 22
OUT2 DELAY SETTING = 22
OUT3 DELAY SETTING = 22
NOTES:
1.SYNC RISING EDGE (NOT SHOWN) ALIGNS RDIV OUPUT TO REF INPUT
2.RDIV = 2
3.PDIV CYCLE = 500ps (NOT SHOWN)
16 | May 2016 : LT Journal of Analog Innovation
interface that has undergone two major
revisions since its original 2006 specification. The original goal of JESD204 was
to simplify and lower the cost of the
digital interface by reducing the number
of converter output pins, FGPA pins and
the board area consumed by routing
multiple ADCs to an FPGA. The latest
revision, JESD204B, added the ability to
establish deterministic latency between
a logic device and the data converters.
Over the past few years, a large percentage of the new converter ICs and FPGAs
have adopted the JESD204B interface.
To enable deterministic latency,
JESD204B added two new subclasses,
subclass 1 and subclass 2. Subclass 1 is
the preferred method when converter
clocks are faster than 500Msps.
JESD204B subclass 1 added the alignment signal SYSREF. From the clock IC’s
perspective, SYSREF is phase aligned to
the clock signal, and can range from a
single pulse to several pulses at an integer
multiple of the converter clock period.
As a result, many existing clock devices
did not have the divider range to support
the JESD204B clock and SYSREF signals.
design features
Over the past few years a large percentage of the new converter ICs and FPGA
have adopted the JESD204B interface. The LTC6951 is JESD204B subclass
1 capable, due to the LTC6951 output divider ranging from 1 to 512.
SYNC–
LTC2123
14-BIT ADC
CLK
REF
OUT0
Figure 9. LTC6951 EZSync: JESD204B subclass 1 example
SYSREF
OUT2
OUT3
LTC6951
REF CLK
SYNC–
LANE0
LANE1
FPGA CLK
1:3
BUFFER
FPGA
SYSREF
MGMT CLK
OUT4
OUT1
CLK
SYNC–
SYSREF
LTC2123
14-BIT ADC
SYNC–
LTC2123 #2
14-BIT ADC
LANE2
LANE3
SYNC–
CLK
REF
SYNC
SYSREF
FPGA CLK
MGMT CLK
OUT2
OUT4
LTC2123 #2
14-BIT ADC
REF CLK
LTC6951 SYNC
(ALIGNED TO
REF CLK)
REF
SYNC
FPGA
SYSREF
OUT0
LTC6951 OUT3
#1
OUT1
SYNC–
The LTC6951 is JESD204B subclass 1
capable, due to the LTC6951 output
divider range extending from 1 to 512.
In addition to Figure 7’s EZ204Sync
example, Figure 9 provides an EZSync
Standalone example of a LTC6951 clocking two JESD204B converters. Figure 10
shows a ParallelSync example of multiple
LTC6951s clocking several JESD converters.
SYNC–
LANE0
LANE1
CLK
SYSREF
CLK
SYSREF
SYNC–
LANE2
LANE3
OUT0
LTC6951 OUT3
#2
OUT1
OUT2
OUT4
LTC2123 #3
14-BIT ADC
SYNC–
LANE4
LANE5
SYNC–
REPEAT LTC6951 AND ADC SCHEMATIC
SYNC–
LTC2123 (2N – 2)
14-BIT ADC
REF
Figure 10. LTC6951 ParallelSync: JESD204B subclass 1 example
SYNC
CLK
SYSREF
CLK
SYSREF
SYNC–
LANE(4N – 6)
LANE(4N – 5)
OUT0
LTC6951 OUT3
#N
OUT1
OUT2
OUT4
LTC2123 (2N – 1)
14-BIT ADC
SYNC–
LANE(4N – 4)
LANE(4N – 3)
SYNC–
May 2016 : LT Journal of Analog Innovation | 17
For initial evaluation, the LTC6951Wizard provides
register settings files that are based off the LTC6951
data sheet examples and typical application circuits.
TOOLS
The LTC6951 demo board (www.linear.
com/ product/LTC6951#demoboards) and
the LTC6951Wizard™ greatly simplify
evaluation and design. These tools can:
•Read/write to the LTC6951
SPI registers (Figure 11)
•Calculate registers settings and
design loop filters based on a
frequency plan (Figure 12)
•Simulate time and frequency domain
response based on register settings
and loop filter design (Figure 12)
Figure 11. LTC6951Wizard settings
18 | May 2016 : LT Journal of Analog Innovation
For initial evaluation, the LTC6951Wizard
provides register settings files that
are based on the LTC6951 data sheet
examples and typical application
circuits. To evaluate a custom frequency
plan, the LTC6951Wizard provides a
Help file with step-by-step examples
using the LTC6951Wizard to calculate
register settings, design the loop filter
and program the LTC6951 SPI registers. Download the LTC6951Wizard at
www.linear.com/LTC6951Wizard.
CONCLUSION
The LTC6951 produces clock frequencies
up to 2.7GHz with the lowest wideband
noise floor in the industry for a clock
distribution device. This allows the
LTC6951 to directly clock high speed ADCs
with very challenging SNR and clockto-clock skew targets. The LTC6951’s
variety of synchronization methods
allows designers to optimize for ease
of synchronization, clock jitter and the
number of clocks required. The LTC6951
supports JESD204B subclass 1 converter
clock schemes. To further simplify
design, the LTC6951Wizard is provided to
guide the user through design, simulation and evaluation of the LTC6951. n
design features
To evaluate a custom frequency plan, the LTC6951Wizard provides a Help file
with step-by-step examples using the LTC6951Wizard to calculate register
settings, design loop filter and program the LTC6951 SPI registers.
Figure 12. LTC6951Wizard loop filter design and simulation
May 2016 : LT Journal of Analog Innovation | 19
1.5A, Negative Regulator Expands Family of CurrentReference Linear Regulators
Dawson Huang
The LT3080, introduced in 2007, represented a new linear regulator architecture featuring
a current source as reference and a voltage follower for the output amplifier. This new
architecture has a number of advantages, including easy regulator paralleling for increased
output current and operation down to zero output voltage. Since the output amplifier always
operates at unity gain without a resistor-setting divider, bandwidth and absolute regulation
are constant across the output voltage range. Transient response is independent of output
voltage and regulation can be specified in millivolts rather than as a percent of output.
Table 1 summarizes the family of devices
that use this architecture. The LT3091,
the latest addition to this family, is a 1.5A
low dropout negative linear regulator
featuring adjustable current limit and
current monitor. The LT3091 is similar
to the other negative linear regulator in
Figure 1. 1.5A, negative linear
regulator with current limitation and
monitor
0.1µF
LT3091
the family, the LT3090, but with more
than double the LT3090’s current rating.
The LT3091 is useful in high current,
negative voltage applications requiring
low noise or precision output. It features
fast transient response, high PSRR and
low output noise. Low dropout helps
HOW IT WORKS
TO ADC (IMON)
RSET
49.9k
RMON
6.65k
SET
GND
10µF
VOUT
–2.5V
MAX IOUT
1.5A
IMONP OUT
+
–
10µF
VIN
–3V TO –10V
50µA
IN
ILIM
SHDN
IMONN
3.3V
0.1µF
RLIM 5k
Table 1. Some of Linear’s regulators featuring the current reference architecture
LT3091
LT3090
LT3081
LT3080
Output current
1.5A
600mA
1.5A
1.1A
I SET
−50µA
−50µA
50µA
10µA
Adjustable current limit/current monitor
Yes/Yes
Yes/Yes
Yes/Yes
No/No
LDO (low dropout)
Yes
Yes
No
Yes
Positive/Negative voltage
Negative
Negative
Positive
Positive
20 | May 2016 : LT Journal of Analog Innovation
keep it from overheating when supporting loads up to 1.5A. Built-in protection includes reverse output protection,
internal current limit with foldback and
thermal shutdown with hysteresis. This
versatile negative regulator architecture
can operate down to zero volts out
and as a negative floating regulator.
The negative output voltage is set with
a −50µ A precision current source driven
through a single resistor RSET from ground
to the SET pin. The internal follower
amplifier forces the output voltage to
match the negative voltage of the SET
pin. With this architecture, all of the
internal operating current flows in from
the output pin. Only a 20µ A load is
required to maintain regulation at all
output voltages. Figure 1 shows the basic
hookup for the LT3091. It provides 1.5A
of output current, can be adjustable to
zero output voltage, and features both
positive and negative monitors for output
current. It is also reverse protected, when
output voltage is lower than input.
The current limit can be reduced
below 1.5A by connecting an external
resistor RLIM between ILIM and IN pins,
design features
This regulator is easy to parallel to increase
output current. It can be used for power supplies
capable of sinking and sourcing current.
as shown in Figure 1. This function
can effectively protect the load and
limit the temperature of the IC.
0.1µF
LT3091
U1
With 3.3V feeding the IMONN pin, the
IMONP pin sources current equal to ¼000
of the output current. This current source
is measured by tying a resistor, RMON , to
ground in series with the current source
and reading the voltage across the resistor. With the IMONP pin tied to VIN , the
IMONN pin sinks current equal to ½000 of
the output current. In this way, positive or
negative output current can be monitored
with minimal components, no additional
sense resistors or amplifiers required.
PARALLELING DEVICES FOR MORE
CURRENT
Paralleling LT3091s is easy with this
new current source reference regulator.
Paralleling is useful for increasing output
current or spreading heat. Since the
LT3091 is set up as a voltage follower,
tying all the SET pins together makes
the outputs the same voltage. If the
outputs are at the same voltage, only
a few milliohms of ballast, ROUT1,2 , are
required to allow them to share current.
Figure 2 shows a schematic of two
LT3091s paralleled to obtain 3A output.
The set resistor, RSET, now has twice
the set current flowing through it, so
the output is −100µ A times RSET. The
10m Ω output resistors, ROUT1,2 ensure
ballasting at full current. There is no
limit to the number of devices that
can be paralleled for higher current.
RSET
24.9k
1%
22µF
SET
GND
IMONN OUT
ROUT1
10mΩ
+
VOUT
–2.5V
MAX IOUT
3A
–
22µF
VIN
–3.3V
50µA
IN
IMONP
SHDN
ILIM
5k
GND
LT3091
U2
SET
IMONN OUT
ROUT2
10mΩ
+
Figure 2. 3A negative linear
regulator with paralleled
LT3091
–
50µA
IN
SHDN
IMONP
ILIM
5k
Figure 3. Thermal
performance of two
paralleled LT3091s
U1
52°C
U2
53°C
May 2016 : LT Journal of Analog Innovation | 21
Figure 3 shows the thermal distribution
of the design of Figure 2—U1 and
U2 reach similar temperatures, indicating equally shared current.
and quiet solution. Figure 5 shows the
transient response of the two output
voltages. Figure 6 shows the thermal
performance of the entire system.
LOW NOISE POSITIVE-TO-NEGATIVE
CONVERTER
LOW NOISE POSITIVE AND NEGATIVE
POWER SUPPLY
Inverting converters generate a negative
voltage from a positive input, and feature
low output ripple. If combined with a
high bandwidth LDO such as the LT3091,
the overall converter can have very high
transient response with even lower noise.
A high current positive-to-positive-andnegative converter can be built with a
positive 1.5A LT3081 linear regulator and
its negative 1.5A linear counterpart, the
LT3091. The LT8582 is a dual-channel PWM
DC/DC converter with internal switches in
an available 7mm × 4mm DFN package.
It can generate both a positive and a
negative output from a single input.
Figure 4 shows a low noise coupledinductor positive-to-negative converter.
The inverting converter is based on
LT3581, a PWM DC/DC converter with
built-in power switch. Its 4mm × 3mm
DFN package and tiny externals can be
combined with the LT3091 in a compact
Figure 7 shows a 1.5A 12V-to-±3.3V
low noise power supply using
the LT8582, LT3081 and LT3091.
Figure 8 shows the transient response
Figure 4. 1.5A low noise and fast transient
positive-to-negative converter
C2
1µF
L1A
3.3µH
VOUT1
–5V
1.5A
L1B
3.3µH
CONCLUSION
The LT3091 is a 1.5A, low dropout, current
reference negative linear regulator. This
regulator is easy to parallel to increase
output current. It also features fast transient response, high PSRR and low output
noise, making it ideal as a post regulator.
It can be used for power supplies capable
of sinking and sourcing current. n
0.1µF
49.9k
1%
LT3091
SET
10µF
1206
D1
SW1 SW2
VIN
C1
22µF
C1: 22µF, 25V, X7R, 1210
C2: 1µF, 50V, X7R, 1206
C3: 22µF, 16V, X7R, 1210
D1: CENTRAL CMSH3-40FL
L1: COILCRAFT MSD7342-332MLB
This setup can be used as an operational
amplifier power supply—where a high
speed operational amplifier requires a low
noise, high speed ±3.3V power supply.
•
•
VIN
12V
of the negative rail. Figure 9 shows the
temperature of the entire system.
100k
124k
LT3581
FB
SHDN
GATE
FAULT
CLKOUT
RT
VC
SYNC
SS
GND
C3
22µF
0.1µF
IMONN OUT
+
60.4k
56pF
GND
6.8k
–
10µF
1206
50µA
IN
3.3nF
SHDN
IMONP
ILIM
5k
Figure 5. Transient response for positive-to-negative
converter
VOUT1
100mV/DIV
(AC COUPLED)
Figure 6. Thermal image
for positive-to-negative
converter
LT3581 54°C
D1: 54°C
VOUT2
100mV/DIV
(AC COUPLED)
L1: 64°C
LT3091 83°C
IOUT
−1A/DIV
100µs/DIV
22 | May 2016 : LT Journal of Analog Innovation
VOUT2
–2.5V
MAX IOUT
1.5A
design features
The LT3091 is useful in high current, negative voltage applications requiring
low noise or precision output. It features fast transient response, high
PSRR and low output noise, making it ideal as a post regulator. Low
dropout helps keep it from overheating when supporting 1.5A loads.
Figure 7. 12V to ±3.3V low noise power supply
10µF
C1
2.2µF
L1A
4.7µH
VOUT1'
5V
D1
•
VIN
12V
CIN1
22µF
SWA1
SWB1
•
VIN1
100k
PG1
LT8582
100k
CLKOUT1
RT1
13k
IMON
ILIM
1.5nF
0.1µF
66.5k
1%
0.1µF
66.5k
1%
22µF
GND
115k
RT2
SHDN2
SS2
VIN2
VC2
SWA2 SWB2
2.2nF
0.1µF
47pF
22µF
18.7k
C2
2.2µF
•
SET
GND
IMONN OUT
+
VOUT2
–3.3V
MAX IOUT
1.5A
–
L2B
4.7µH
•
CIN1, CIN2: 22µF, 25V, X7R, 1210
COUT1, COUT2: 22µF, 16V, X7R, 1210
C1, C2: 2.2µF, 50V, X7R, 1206
D1, D2: CENTRAL CMSH3-40FL
L1, L2: WÜRTH WE TDC 74489440047
LT3091
COUT2
22µF
×2
60.4k
FBX2
Figure 8. Transient response at VOUT2 load transient
TEMP
VOUT2'
3.3V
MAX IOUT
1.5A
47pF
0.1µF
GATE2
CIN2
22µF
OUT
SET
115k
PG2
L2A
4.7µH
–
COUT1
22µF
×2
VC1
SS1
SYNC2
+
45.3k
GATE1
SYNC1
CLKOUT2
ISET
50µA
L1B
4.7µH
FBX1
SHDN1
IN
LT3081
D2
VOUT1
–5V
50µA
10µF
IN
SHDN
IMONP
ILIM
5k
Figure 9. Thermal image
for 12V to ±3.3V low noise
power supply
LT3081 75°C
VOUT1
100mV/DIV
(AC COUPLED)
VOUT2
100mV/DIV
(AC COUPLED)
LT8582 81°C
IOUT
−1A/DIV
LT3091 83°C
100µs/DIV
May 2016 : LT Journal of Analog Innovation | 23
What’s New with LTspice IV?
Gabino Alonso
Blog by Engineers, for Engineers
www.linear.com/solutions/LTspice
NEW VIDEO: “BEHAVIORAL VOLTAGE
SOURCES” by Simon Bramble
Nearly all circuit simulations require
a voltage source. This video reveals
some of the undiscovered talents of the
not-so-humble LTspice® voltage source,
specifically exploring the power of the
behavioral voltage source. A behavioral
voltage source outputs a voltage according
to any number of circuit parameters, and
it can be used to unleash the real mathematical power of LTspice.
www.linear.com/solutions/6106
SELECTED DEMO CIRCUITS
For a complete list of example simulations utilizing Linear Technology’s devices,
please visit www.linear.com/democircuits.
Buck Regulators
—Follow @LTspice at www.twitter.com/LTspice
—Like us at facebook.com/LTspice
• LTM®4622: Dual step-down regulator
(3.6V–20V to 3.3V & 1.2V at 2.5A)
www.linear.com/solutions/5847
• LTM4675: Paralleled µModule
buck regulator with digital
interface (10V–14V to 1V at 72A)
www.linear.com/solutions/5833
Boost Regulators
• LT3095: Dual low noise, low ripple bias
generator (3V–20V to 5V & 15V at 50m A)
www.linear.com/solutions/6001
• LT8331: 120V boost converter
(36V–72V to 120V at 60m A)
www.linear.com/solutions/6022
SEPIC Regulator
• LT8331: 48V SEPIC converter
(36V–72V to 48V at 165m A)
www.linear.com/solutions/6019
• LT8602: Automotive quad buck regulator
(5.5V–42V to 5V at 1.5A, 3.3V at
2.5A, 1.8V at 1.8A, 1.25V at 1.8A)
www.linear.com/solutions/5835
• LTC3649: High voltage monolithic
synchronous buck regulator with cable
drop compensation (4V–60V to 5V at 4A)
www.linear.com/solutions/7117
Buck-Boost Regulator
• LTM8055: Paralleled synchronous
buck-boost regulator with accurate
current limit (7V–36V to 12V at 12A)
www.linear.com/solutions/5617
4-Quadrant Converter
• LT8714: Synchronous four quadrant
converter with power good indication
(10V–14V to −5V to 5V at −5A to 5A)
www.linear.com/solutions/6004
Operational Amplifier
• LTC6268-10: Oscilloscope differential probe
www.linear.com/solutions/6058
SELECTED MODELS
To search the LTspice library for a
particular device model, press F2. Since
LTspice is often updated with new
features and models, it is good practice to
update to the current version by choosing
Sync Release from the Tools menu.
Buck Regulators
• LT8641: 65V, 3.5A synchronous step-down
Silent Switcher® with 2.5µ A quiescent
current www.linear.com/LT8641
• LTM4650: Dual 25A or single 50A
DC/DC µModule regulator
www.linear.com/LTM4650
Inverting Buck Regulator
• LTC7149: 60V, 4A synchronous step-
down regulator for inverting outputs
www.linear.com/LTC7149
Isolated Flyback Converter
• LTM8068: 2.8V to 40V input isolated
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 | May 2016 : LT Journal of Analog Innovation
µModule DC/DC converter with LDO
post regulator www.linear.com/LTM8068
Charge Pumps
• LTC3265: Low noise dual supply with
boost and inverting charge pumps
www.linear.com/LTC3265
design ideas
Hot Swap Controller
Ideal Diode-OR Controller
Comparator
• LTC4281: Hot swap controller
• LTC4371: Dual negative voltage ideal
• LTC6754: High speed rail-to-rail input
diode-OR controller and monitor
www.linear.com/LTC4371
comparator with LVDS compatible
outputs www.linear.com/LTC6754
with I2C compatible monitoring
www.linear.com/LTC4281
PoE Powered Device
LT4276A:
LTPoE++®/PoE+/PoE PD forward/
flyback controller www.linear.com/LT4276
Op Amp
Voltage Reference
• LT6375: ±270V common mode
• LT6657: 1.5ppm/°C drift, low
voltage difference amplifier
www.linear.com/LT6375
noise, buffered reference
www.linear.com/LT6657 n
Power User Tip
AC ANALYSIS USING THE STEP COMMAND
In LTspice, AC analysis involves computing the AC
complex node voltages as a function of frequency
using an independent voltage or current source as
the driving signal. The small signal analysis results
are plotted in the waveform viewer as magnitude and
phase over frequency.
AC analysis in LTspice has a number of settings: the
x-axis scaling (linear, octave or decade), number of
simulation points and frequency range. For example, if
you want to see how your circuit performs from 100Hz
to 1MHz with 1,000 points per decade you would edit
your simulation command to the following:
.ac dec 1K 100 1Meg
Repeated AC Analysis with Parameter Sweeps
AC analysis usually involves using fixed parameters
to calculate the small signal AC response of a circuit,
but you may want to refine your design by viewing
the response under varying parameters. This can be
accomplished by stepping the parameter of interest
using a .step command. For example, you could sweep
a capacitance logarithmically through the range of
10pF to .5nF with 30 points per octave using the .step
directive (press S to insert a spice directive in the
schematic editor):
.step oct param C 10p .5n 30
The schematic for this case and its resulting waveform
are shown below.
The beauty of single frequency analysis with a .step
command is that the resulting plot shows magnitude
and phase as a function of parameter sweep, not
frequency. Below is the result of a simulation using
a single frequency analysis where the x-axis is the
capacitance sweep as defined in the .step function.
Note that using a .step command with AC analysis
can drastically increase simulation time, so carefully
choose the values, ranges, increments, and frequency
range for each parameter sweep.
Single Frequency Analysis with a Swept Parameter
LTspice offers an elegant solution for holding frequency
constant and performing small signal analysis over
a varying parameter. It is as simple as using the ‘list’
AC Analysis option in the simulation command, and
specifying the frequency at which you want to perform
the analysis, in this case, 1MHz:
.step oct param C 10p .5n 30
.ac list 1Meg
AC analysis commands can be edited using the
Edit Simulation Command dialog.
Happy simulations!
Repeated AC analysis with stepped capacitance
Single frequency analysis with swept capacitance
The x-axis is capacitance as
defined by the .step command.
May 2016 : LT Journal of Analog Innovation | 25
Easy Balanced Load Sharing for Three or Four Supplies,
Even with Unequal Supply Voltages
Vladimir Ostrerov and Chris Umminger
Using multiple small power supplies is often more
economical and more reliable than using a single large
power supply. For instance, separate batteries can be
used for higher reliability. In a multi-supply system, it
is important that the load is equally shared; otherwise,
one supply may attempt to carry the entire load.
This article shows how to easily load balance three
or four supplies by cascading LTC4370 circuits.
The LTC4370 controller enables current
sharing between two supplies with a
modest difference between the output
voltages, as shown in Figure 1. To perfectly
balance the current in both sides, the
controller regulates the gate-source voltage
of an N-channel MOSFET in whichever
side has the higher voltage. This creates
a voltage drop across the MOSFET’s
RDS(ON) plus the current sense resistor.
The LTC4370 can compensate for a voltage
difference between two rails of up to
0.5V. If the voltage difference of the two
supplies is somewhat less than 0.5V, the
Figure 1. The LTC4370 currentbalancing controller enables balanced
load sharing between two supplies,
even when their voltage outputs are
different.
of the total load current equally. The
output voltage at the load is less than
the minimum of the supply voltages V1,
V2 and V3. Because there are two stages
of cascading, it is possible to have as
much as 1V difference between V3 and
V1 or V2, if the difference between V1
and V2 is already at the 0.5V limit.
LTC4370 can regulate its output to match
the lower value rail, set by adding an
appropriate resistor on the RANGE pin.
BALANCING THE LOAD BETWEEN
THREE SUPPLIES WITH TWO
CASCADED LTC4370s
Figure 2 shows a 3-input, 12V system
delivering 10A. Notice that one LTC4370
(U1) performs equal current sharing
between supplies V1 and V2, while the
second LTC4370 (U2) implements a 2:1
relation between the output current of
U1 and the current of a third supply, V3.
Thus, each supply contributes one third
39nF*
0.1µF
NC
VIN1
GATE1
OUT1
VCC
OUT2
EN2 CPO2
VIN2
*OPTIONAL, FOR FAST TURN-ON
2mΩ
11.875V
0.18µF
SUM85N03-06P
+ 25mV –
5A
26 | May 2016 : LT Journal of Analog Innovation
10A
GATE2 COMP
39nF*
11.9V
2mΩ
FETON2
RANGE
•Sense resistor tolerance, worst-case
for 1% resistors is 2% overall.
11.875V
FETON1
LTC4370
GND
LIMITATIONS
•LTC4370 error amplifier input
offset, ±2mV (maximum)
+ 325mV –
SUM85N03-06P
EN1 CPO1
Cascading three LTC4370 controllers
(Figure 2) allows four supplies to share
the load. In the first stage, U1 and U2
force equal sharing between a pair of
supplies, where the output current of U1 is
I12 = I1 + I2 , and the output current of U2
is I34 = I3 + I4 . A third LTC4370, the second
stage, keeps I12 = I34 . Thus, each supply
contributes one fourth of the total load
current. The two stages, as above, allow
the possibility of as much as 1V difference between the four supply voltages.
The main error sources that affect
perfect current sharing are:
5A
12.2V
BALANCING THE LOAD BETWEEN
FOUR SUPPLIES
Sharing error attributed to the error
amplifier input offset decreases with
increasing sense voltage, but power
dissipation increases. For the simple
LTC4370 circuit with two supplies, this
error is expressed as an imbalance in
the supplies’ sharing of current:
design ideas
12.4V
EN1
SUM85N03-06P
∆I = I1 − I2
39nF 50V
EN1 CPO1
VIN1
10k
Using the worst-case errors,
above, the error is:
GATE1
OUT1
VCC
VCC
0.1µF
U1
LTC4370
GND
NC
2mΩ
FETON2
2mΩ
RANGE
10k
VIN2
GATE2 COMP
39nF 50V
12.0V
Figure 2. Two LTC4370s
can be cascaded to
enable current sharing
of three supplies.
 2mV

∆I ≤ 
+ 0.01• ILOAD  [A]
 RSENSE

I12 = I1 + I2 = ILOAD
I2 = ILOAD
OUT2
EN2 CPO2
EN2
I1 = ILOAD
FETON1
For the circuit of Figure 2, where ideal
load sharing means the load is distributed into 1⁄3ILOAD and 2⁄3ILOAD , it is easier
to estimate the worst-case imbalance
via an expression of the maximum and
minimum current of each supply:
CCOMP
0.18µF
RCOMP
15k
SUM85N03-06P
SUM85N03-06P
39nF 50V
EN1
EN1 CPO1
VIN1
10k


2mV
IMAX =  0.672 • ILOAD +
[A]
3.01• RSENSE 

GATE1
OUT1
VCC
VCC
0.1µF
U2
LTC4370
GND
NC
2mΩ
FETON2
4mΩ
VIN2
10k
GATE2 COMP
39nF 50V
12.0V
V1
EN1
SUM85N03-06P
VIN1
GATE1
OUT1
VCC
U1
LTC4370
GND
NC
2mΩ
FETON2
2mΩ
OUT2
VIN2
EN2 CPO2
GATE2 COMP
39nF 50V
Figure 3. Four supplies
can each support an equal
share of a load by using
three LTC4370s in a 2-stage
cascade.
V2
V3
I1 = ¼ILOAD
FETON1
RANGE
10k
By cascading the shared output of one
LTC4370 with another LTC4370, three or
more supplies can be efficiently controlled
to provide equal current to the load. With
errors on the order of the sense resistor
tolerance, the voltage drop is minimal. n
39nF 50V
EN1 CPO1
EN2
CCOMP
0.18µF
RCOMP
15k
SUM85N03-06P
SUM85N03-06P
EN1
EN1 CPO1
OUT1
VCC
VCC
0.1µF
U2
LTC4370
GND
NC
10k
2mΩ
FETON2
2mΩ
VIN2
GATE2 COMP
39nF 50V
V4
GND
I12 = ½ILOAD
FETON1
2mΩ
FETON2
2mΩ
RANGE
VIN2
GATE2 COMP
39nF 50V
ILOAD
I34 = ½ILOAD
OUT2
I3 = ¼ILOAD
FETON1
OUT2
RANGE
EN2 CPO2
EN2
GATE1
U3
LTC4370
EN2 CPO2
10k
GATE1
OUT1
VCC
NC
VIN1
VIN1
10k
39nF 50V
EN1 CPO1
39nF 50V
VCC
0.1µF
SUM85N03-06P
10k
I12 = I1 + I2 = ½ILOAD
I1 = ¼ILOAD
EN2
EN1
CONCLUSION
CCOMP
0.18µF
RCOMP
15k
SUM85N03-06P
10k
VCC
0.1µF
ILOAD
I3 = ILOAD
OUT2
EN2 CPO2
EN2
FETON1
RANGE


2mV
IMIN =  0.328 • ILOAD +
[A]
3.01• RSENSE 

I12 = I1 + I2 = ILOAD
CCOMP
0.18µF
RCOMP
15k
SUM85N03-06P
I4 = ¼ILOAD
I34 = I3 + I4 = ½ILOAD
CCOMP
0.18µF
RCOMP
15k
SUM85N03-06P
May 2016 : LT Journal of Analog Innovation | 27
Design Once; Use Twice: Monolithic SEPIC/Boost Regulators
with Wide VIN Range Satisfy Requirements of Both Consumer
and Commercial Vehicles
Molly Zhu
Automobile manufacturers continually add electronic control units (ECUs) to support
increasing numbers of performance, comfort and safety features. ECU power either
comes from a single lead-acid battery in consumer vehicles, or from two batteries in
commercial vehicles. Ideally, an ECU can run off either, enabling a single design for
both consumer and commercial vehicles. This requires that ECU power ICs support
an input range covering both configurations—namely 3.5V to 60V. Furthermore,
the power ICs should feature ultralow quiescent current, preserving the vehicle’s
battery run time when the engine is off, but always-on systems remain engaged.
The LT8495 and LT8494 are high voltage
switching regulators that meet these
requirements when configured as
SEPIC or boost converters. Both parts
operate over 2.5V to 60V input, and
have low quiescent current to extend
the battery life. The quiescent current
of the LT8495 is 9µ A, and is 7µ A for the
LT8494. The parts are available in 20-lead
QFN and 20-lead TSSOP packages.
supply voltage is monitored by poweron reset, and the software/hardware
activities are supervised by watchdog
timers. These functions are integrated
in the LT8495, simplifying designs with
enhanced safety and reliability.
DUAL SUPPLY PINS
The input voltage of the LT8494/LT8495
can be as high as 60V for SEPIC topologies, and 32V for boost circuits with the
60V ride-through voltage. The internal
power switch driver must be in the
2.4V~34V (typical) range to enable the
LT8494/LT8495, but the minimum operating VIN range can be reduced to 1V. The
The LT8494 and LT8495 are similar, but
the LT8495 adds power-on reset and
watchdog timers. It is designed specifically
for microcontrolled applications, where
reliability and safety are critical. The
Figure 1. The LT8494 in a 750kHz, 48V boost converter
100
SW
VIN
SWEN
PG
RT
93.1k
SS
10pF
VOUT
48V
0.5A
FB
LT8494
GND BIAS
0.2µF
25.5k
C2
4.7µF
×2
95
EFFICIENCY (%)
D1
1M
C1
2.2µF
A typical application of a boost converter
using the LT8494 is shown in Figure 1.
The BIAS pin is connected to ground
instead of the output since the input
Figure 2. Efficiency of the circuit in Figure 1
L1
22µH
VIN
16V TO 32V
integrated power switch drivers can
operate from either of two supplies: VIN
or BIAS. This allows the part to optimize
efficiency and reduces the minimum
input voltage requirement. The LT8494/
LT8495 automatically chooses the lower
supply of the two, provided it is in the
operation range. This selection is made
on-the-fly as VIN or BIAS voltages change.
After initial start-up, the part can draw
current from BIAS if it is lower than VIN .
90
85
80
75
C1: 2.2µF, 50V, X5R, 1206
C2: 4.7µF, 100V, X7R, 1210
D1: ONSEMI MBRA2H100
L1: WÜRTH LHMI 74437349220
28 | May 2016 : LT Journal of Analog Innovation
70
0
100
200
300
400
LOAD CURRENT (mA)
500
design ideas
RSTIN
1.1V
WDI
WDO
RST
tRST
tUV
tWDL< t < tWDU
t < tWDL
t > tWDU
tRST
tRST
tWDU = WATCHDOG UPPER BOUNDARY PERIOD, APPROXIMATELY 31 RAMPING CYCLES ON CWDT PIN
tWDL = WATCHDOG LOWER BOUNDARY PERIOD, APPROXIMATELY 1 RAMPING CYCLE ON CWDT PIN
tUV = TIME REQUIRED TO ASSERT RST LOW AFTER RSTIN
GOES BELOW ITS THRESHOLD, APPROXIMATELY 23µs
tRST = PROGRAMMED RESET PERIOD
(a)
(b)
Figure 3. POR (a) and watchdog (b) timing
voltage is always lower than the output.
The efficiency is given in Figure 2. At
very light load, the efficiency of the
LT8494 is slightly higher than that of
the LT8495 because the LT8494 is not
supporting a watchdog function.
WATCHDOG TIMER AND POR
FUNCTIONS
The LT8495 is similar to the LT8494, but
it adds integrated power-on reset (POR)
and watchdog timer functions to enhance
system safety in automotive applications.
The POR monitors the supply voltages,
while the watchdog timer monitors the
software and hardware functions.
by the RSTIN pin, and the watchdog
timer supervises the microcontroller.
The LT8495’s watchdog timer includes
an independent enable pin (WDE), and
can operate without the VIN supply. If
the time between the negative edges
on the WDI is too long or too short,
the WDO pin is pulled low for the reset
delay time, tRST, before it is released.
The window time of WDI can be
programmed through the cap on CWDT
pin. The timing diagrams of the POR and
watchdog timer are shown in Figure 3.
The LT8494 and LT8495 are monolithic
boost/SEPIC switching regulators with
input voltage ranges of 1V to 60V after
start-up. Both parts can automatically
select the lower supply pins, VIN or BIAS,
to improve efficiency. The LT8495 features
an integrated power-on reset and a watchdog timer to monitor the microcontroller’s
activity. Their wide input voltage ranges,
high efficiency, low quiescent current and
programmable timing make them ideal for
industrial and automotive applications. n
Figure 4 shows the LT8495 configured as
a SEPIC converter with a 3V–60V input
voltage and 5V output. The max load
current increases with the input voltage
until reaching the full load current of 1A at
12V input. The output voltage is monitored
The LT8495 monitors the output via the
RSTIN pin voltage. During normal operation, if the voltage of the RSTIN is below its
threshold, the RST pin is asserted low. Once
the RSTIN rises above its threshold, the RST
Figure 4. The LT8495 in a 450kHz, 5V
output SEPIC converter with POR
and watchdog timer
pin is released after the reset delay time.
The reset delay time, tRST, is programmable through the cap on the CPOR pin.
L1
15µH
VIN
3V TO 60V
•
C3
2.2µF
C1
2.2µF
•
SW
SWEN
1nF
SS
RT
4.7nF
1µF
169k
1M
4.7pF
BIAS
RSTIN
FB
VIN
CPOR
VOUT
5V
0.4A (VIN = 3V)
0.6A (VIN = 5V)
1.0A (VIN > 12V)
D1
L2
15µH
CWDT
CONCLUSION
C2
47µF ×2
8.87k
316k
GND
LT8495
WDO
WDE
RST
WDI
µC
C1, C3: 2.2µF, 100V, X5R, 1206
C2: TAIYO YUDEN, EMK325BJ476MM-T
D1: ONSEMI MBRA2H100
L1, L2: COILTRONICS DRQ125-150-R
May 2016 : LT Journal of Analog Innovation | 29
New Product Briefs
±270V COMMON MODE DIFFERENCE
AMPLIFIER FEATURES 97dB MIN
CMRR, ±35PPM MAX GAIN ERROR
The LT6375 is a unity-gain difference
amplifier with integrated precision
matched resistors, which precisely level
shifts and buffers small difference signals
while rejecting up to ±270V common
mode. The A-grade version achieves
unprecedented performance: CMRR is
97dB (min), initial gain error is 35ppm
(max), gain drift is 1ppm/°C (max) and
gain nonlinearity is 2ppm (max) with a
common mode divide ratio of 25:1. The
common mode divide ratio is selectable
from 7:1 to 25:1, enabling the designer to
select the ratio with the best performance
for a given common mode input range.
At the heart of the LT6375 is a high
precision Over-The-Top® amplifier which
operates with inputs both within and
above the 3.3V to 50V supply voltage.
This permits the combination of a wide
input range and low voltage supply.
Use of a low voltage supply limits
power consumption and protects downstream circuitry from high voltage.
“The LT6375 combines a high precision, wide voltage range Over-The-Top
amplifier with configurable precision
matched resistors,” says Maziar Tavakoli,
Design Manager, Signal Conditioning
Products. “With seven different divider
ratios to choose from, precision, noise
30 | May 2016 : LT Journal of Analog Innovation
and speed can be optimized for specific
input range requirements. For example,
if the input common mode range is
±80V, a resistor divider ratio of seven
can be selected to achieve lower noise,
lower offset and wider bandwidth than
is achievable with a ratio of 20.”
For noise-sensitive applications, the
LT8602 utilizes its pulse-skipping mode
to minimize switching noise and meet
the CISPR25, Class 5 EMI requirements.
Switching frequency can be programmed
from 250kHz to 2.2MHz and is synchronizable throughout this range.
The LT6375 includes many other useful
features, including rail-to-rail outputs,
low supply current and a shutdown
mode. It is available in a 4mm × 4mm
12-lead DFN and a 4mm long MSOP
package with 12 leads. Both packages include skipped leads for extra
spacing of high voltage input signals.
The LT8602’s 60ns minimum on-time
enables 16V VIN to 0.8V VOUT stepdown conversions while switching at
2MHz , enabling designers to avoid
critical noise-sensitive frequency
bands such as AM radio, while using
a very compact solution footprint.
42V QUAD SYNCHRONOUS
STEP-DOWN DC/DC CONVERTER
DELIVERS 93% EFFICIENCY &
OPERATES FROM 3V TO 42V INPUTS
The LT8602 is a 42V input capable, high
efficiency quad output synchronous
monolithic step-down switching regulator. Its quad channel design combines
two high voltage 2.5A and 1.5A channels
with two lower voltage 1.8A channels
to provide four independent outputs,
delivering voltages as low as 0.8V.
Its synchronous rectification topology
delivers up to 93% efficiency while
Burst Mode® operation keeps quiescent
current under 30µ A (all channels) in
no-load standby conditions, making
it ideal for always-on systems.
Its 3V to 42V input voltage range makes
it ideal for automotive applications that
must regulate through cold-crank and
stop-start scenarios with minimum input
voltages as low as 3V and load dump
transients in excess of 40V. Each channel
of the LT8602 maintains a minimum
dropout voltage of only 200mV (at 1A)
under all conditions, enabling it to excel
in scenarios such as automotive coldcrank. Programmable power-on reset
and power good indicators for each
channel ensure overall system reliability.
The LT8602’s 40-lead thermally enhanced
6mm × 6mm QFN package and high
switching frequency keep external inductors and capacitors small, providing a
compact, thermally efficient footprint.
product briefs
5W AutoResonant WIRELESS POWER
TRANSMITTER FEATURES FOREIGN
OBJECT DETECTION, COMPLETES
LINEAR CHARGING SOLUTIONS
The LTC4125 is a wireless power transmitter that complements Linear’s wireless
receiver ICs in wireless charging solutions.
The LTC4125 is a high performance
monolithic full bridge resonant driver,
capable of delivering up to 5W of power
wirelessly to a companion receiver. It functions as the transmit circuit component in
a complete wireless power transfer system
comprised of transmit circuitry, transmit
coil, receive coil and receive circuitry.
The LTC4125 wireless power transmitter
improves on a basic transmitter by
providing three key features:
•The AutoResonant™ function maximizes
available receiver power
•The optimum power search algorithm
maximizes overall wireless power system
efficiency
•Foreign object detection (FOD) ensures
safe and reliable operation when
working in the presence of conductive
foreign objects.
The LTC4125 automatically adjusts its
drive frequency to match the LC network
resonant frequency. AutoResonant switching enables the device to deliver maximum
power from a low voltage input supply
(3V to 5.5V) to a tuned receiver such as
Linear’s LTC4120 wireless receiver and
battery charger via loosely coupled coils.
Wireless power receivers can also be
designed with the LTC4071 shunt battery
charger or the LT3652HV multi-chemistry
battery charger. To optimize system
efficiency, the LTC4125 employs a periodic
transmit power search and adjusts the
transmission power based on the receiver
load requirements. The device stops
delivering power in a fault condition, or
if a conductive foreign object is detected.
It is surprisingly easy to design an entirely
analog wireless power system using the
LTC4125. The transmit power optimization
and foreign object detection features in
the LTC4125 require no direct communication between the transmitter and receiver
circuits. Even without digital communication, the LTC4125 can work over a wide
range of transmit-to-receive coil coupling
factors—no complicated signal processing hardware and software is needed.
The LTC4125 includes a programmable
maximum current limit and an NTC input
as additional means of foreign object
and overload protection. Applications
include handheld instruments, industrial/
military sensors for harsh environments,
portable medical devices and electrically
isolated devices. LTC4125-based systems
enable robust, standalone solutions
capable of large transmission distances
up to 10mm, which also tolerate poor
coil coupling due to misalignment.
The LTC4125 is housed in a low profile
(0.75mm) 20-pin 4mm × 5mm QFN
package with backside metal pad for
excellent thermal performance. n
33nF
DR1
10µF
4V
TO
5.5V
VIN
100mΩ
412k
2.21k
1µF
100k
100k
47µF
x2
DSTAT
100k
IN
DTH
STAT
10nF
DFB
DC1
PTH2
100k
100V
FB
EN
IMON
10nF
LRX
47µH
150k
CTD
CTS
470pF
GND
0.1µF
L1
15µH
LTC4120-4.2
CHGSNS
FAULT
VIN
PTH1
A complete wireless battery charging system
using LTC4125 AutoResonant wireless power
transmitter in combination with the LTC4120,
forming a 200mA single cell Li-ion battery
charger. The LTC4125 drives a 24μH transmit
coil at 103kHz, with 530mA input current
threshold, 119kHz frequency limit and 41.5°C
transmit coil surface temperature limit.
10nF
SW2
IS+
47µF
BOOST
CTX
100nF
LTC4125
QR1
DHC
IN
SW
RNTCTX
8.45k
IS–
10.2k
RUN
SW1
PTHM
59.0k
M1
RC
1k
24.9k
AIR GAP
3mm
TO
LTX 10mm
24µH
NTC
DC
1.4M
10k
IN1 IN2
FTH
4.32k
DFLZ30
DR2
BAT
CHRG
BATSNS
PROG
GND FREQ INTVCC
NTC
CFB1
0.1µF
7.68k
6.04k
10k
2.2µF
RNTCRX
+
SINGLE
CELL
Li-Ion
BATTERY
PACK
4.7nF
LTX: WT505090-20K2-A10-G
CTX: C3216C0G2A104J160AC
CFB1: HMK107BJ104KA-T
DC1: CDBQR70
DSTAT: LTST-C193KGKT-5A
DFB: BAS521-7
RNTCTX: NTHS0603N02N1002J
RED INDICATES HIGH VOLTAGE PARTS
DR1, DR2, DR3: DFLS240L
DC: BZT52C13
M1: Si7308DN
QR1: PMBT3904M
RNTCRX: NTHS0402N02N1002F
LRX: PCB COIL AND FERRITE: B67410-A0223-X195
OR 760308101303
L1: LPS4018-153ML
May 2016 : LT Journal of Analog Innovation | 31
highlights from circuits.linear.com
LTC7130 HIGH EFFICIENCY, 1.5V/15A STEP-DOWN
CONVERTER WITH VERY LOW OUTPUT RIPPLE
The LTC7130 is a current mode synchronous stepdown monolithic converter that can deliver up to
20A continuous load current. It employs a unique
architecture which enhances the signal-to-noise
ratio of the current sense signal, allowing the use
of a very low DC resistance power inductor to
maximize efficiency in high current applications.
This feature also reduces the switching jitter
commonly found in low DCR applications. The
LTC7130 includes a high speed differential
remote sense amplifier and a programmable
current sense limit that can be selected from
10mV to 30mV to set the output current limit
up to 20A. In addition, the DCR temperature
compensation feature limits the maximum
output current precisely over temperature.
www.linear.com/solutions/7216
VIN
5V TO
20V
2.2Ω
220µF
1µF
10µF
x2
4.7µF
SVIN
INTVCC ILIM ITEMP
BOOST
VIN
PINS NOT USED
IN THIS CIRCUIT:
EXTVCC
PGOOD
CLKOUT
0.22µF
RUN
SW
MODE/PLLIN
SNSD+
ITH
3.3nF
CMDSH3
220pF
26.1k
LTC7130
30.1k
COUT
470µF
×4
220nF
SNSA+
VFB
20k
2.49k
220nF
FREQ
121k
VOUT
1.5V
20A
SNS–
TK/SS
0.1µF
0.72µH,
DCR = 1.3mΩ,
744325072
DIFFOUT
SGND GND
DIFFN
499Ω
DIFFP
LTM8064
VIN
8.5V
TO 58V
VIN
+
OPTIONAL
INPUT
PROTECTION
VOUT
15µF
100V
STACK TWO LTM8064s TO CHARGE AND ACTIVELY
BALANCE SUPERCAPACITORS (OR BATTERIES)
The LTM®8064 is a 58V input, 6A, constant-voltage,
constant current (CVCC), step-down μModule (power
module) regulator. Included in the package are the
switching controller, power switches, inductor and
support components. Operating over an input voltage
range of 6V to 58V, the LTM8064 supports an output
voltage range of 1.2V to 36V. CVCC operation allows
the LTM8064 to accurately regulate its output current
up to 7A when sourcing and 9.1A when sinking over
the entire output range. The output current can be set
by a control voltage, a single resistor or a thermistor.
To set the switching frequency, simply place a resistor
from the RT pin to ground. A resistor from FB to ground
sets the output voltage. Only the bulk input and output
filter capacitors are required to finish the design.
www.linear.com/solutions/7150
2x4.7µF
CTRL1
MODE
SYNC
CTRL2
VREF
RT
GND
2.5V
SUPERCAP
15.0k
PINS NOT USED IN THIS CIRCUIT:
SS, IOUTMON, PGOOD
LTM8064
VIN
15µF
100V
100µF
FB
196k
225kHz
+
VOUT
RUN
VOUT
RUN
100µF
2x4.7µF
MODE
CTRL1
SYNC
CTRL2
2.5V
SUPERCAP
VREF
RT
196k
225kHz
GND
FB
15.0k
PINS NOT USED IN THIS CIRCUIT:
SS, IOUTMON, PGOOD
LTM4632 3.6V TO 15V INPUT, 1.5V/3A VDDQ, 0.75V/±3A VTT AND 10mA VTTR DESIGN
The LTM4632 is an ultrathin triple output step-down μModule (power module)
VIN
regulator to provide complete power solution for DDR-QDR4 SRAM. Operating
3.6V TO 15V RAIL
from a 3.6V to 15V input voltage, the LTM4632 supports two ±3A output rails, both
sink and source capable, for VDDQ and VTT, plus a 10mA low noise reference
VTTR output. Both VTT and VTTR track and are equal to VDDQ/2. Housed in a
6.25mm × 6.25mm × 1.82mm LGA package, the LTM4632 includes the switching
controller, power FETs, inductors and support components. Alternatively, the
power module can be configured as a two phase single ±6A output VTT. Only a
few ceramic input and output capacitors are needed to complete the design.
www.linear.com/solutions/7189
22µF
25V
VDDQ
22µF
4V
PGOOD1 PGOOD2
VOUT1
VIN
RUN1
RUN2 LTM4632 VOUT2
INTVCC
VTTR
SYNC/MODE
FB1
COMP1
TRACK/SS1
VDDQIN
22µF
4V
VTT
0.65V, ±3A
VTTR
0.65V, 10mA
COMP2
GND
VDDQ
1.3V, 3A
52.3k
L, LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode, Dust Networks, LTPoE++, LTspice, Over-the-Top, Silent Switcher and µModule are registered trademarks, and AutoResonant, ClockWizard, EZParallelSync, EZSync, EZ204Sync,
LTC6951Wizard, ParallelSync and SmartMesh IP are trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. © 2016 Linear Technology Corporation/Printed in U.S.A./68.5K
Linear Technology Corporation
1630 McCarthy Boulevard, Milpitas, CA 95035
(408) 432-1900
www.linear.com
Cert no. SW-COC-001530