December 2009 - 1.2A Buck Converters Draw Only 2.8µA When Regulating Zero Load, Accept 38VIN or 55VIN 

LINEAR TECHNOLOGY
DECEMBER 2009
IN THIS ISSUE…
COVER ARTICLE
1.2A Buck Converters Draw Only 2.8µA
When Regulating Zero Load, Accept
38VIN or 55VIN......................................1
John Gardner
Linear in the News...............................2
DESIGN FEATURES
775 Nanovolt Noise Measurement
for a Low Noise Voltage Reference
............................................................6
Jim Williams
Monolithic Synchronous Step-Down
Regulator Sources 3A or Sinks 1.5A
in TSSOP or 3mm × 4mm QFN............10
Genesia Bertelle
Designing a Solar Cell
Battery Charger.................................12
Jim Drew
High Current/High Speed LED Driver
Revolutionizes PWM Dimming.............16
Josh Caldwell
Produce High DC/DC Step-Down Ratios
in Tight Spaces with 30ns Minimum
On-Time Controller in 3mm × 3mm QFN
..........................................................20
Theo Phillips
New Generation of 14-Bit 150Msps
ADCs Dissipates a Third the Power
of the Previous Generation without
Sacrificing AC Performance...............23
Clarence Mayott
Robust, Quiet, Stable Power Supply for
Active Antenna Systems with Built-In
Protection and Diagnostic Capabilities
..........................................................26
Sam Rankin and Steve Knoth
DESIGN IDEAS
.....................................................29–37
(complete list on page 29)
New Device Cameos............................38
Design Tools.......................................39
VOLUME XIX NUMBER 4
1.2A Buck Converters
Draw Only 2.8µA When
Regulating Zero Load,
Accept 38VIN or 55VIN Introduction
In modern battery-powered systems,
extending battery life and intelligently
managing power is paramount. To
conserve power, these systems actively
switch between idle and active states.
The voltage regulators in these systems should be able to do the same.
A regulator must also maintain a wellregulated output voltage during low
current idle states so it can quickly
and automatically adjust to changing
load conditions and provide voltage for
keep-alive functions.
For example, remote monitoring
systems spend most of their time in a
low power idle state, but require bursts
of high power for transmitting data.
Microcontrollers and memory require a
regulated voltage, even when idling, to
hold state. These types of applications
require minimal current consumption
in the idle state to maximize battery
life, and a seamless transition to active
mode when called on to supply several
watts of power.
The LT®3971 and LT3991 are ultralow quiescent current monolithic,
step-down regulators that maintain
high performance at both heavy and
light loads. They draw only 1.7µA of
quiescent current when in light load
situations, but can also source up to
by John Gardner
One way to demonstrate the
low current performance
of the LT3971 is to drive
it from a charged bulk
input capacitor. A 1000µF
capacitor, charged to 16V,
is enough for the LT3971 to
regulate a 3.3V output with
no load for over an hour.
1.2A and include many features of a
high performance 1.2A buck regulator, including programmable fixed
frequency operation, the ability to be
synchronized to an external clock,
soft-start, and a shutdown/enable
pin. The wide input voltage ranges of
these parts—4.3V–38V for the LT3971
and 4.3V–55V for the LT3991—satisfy
the requirements of automotive, industrial, and distributed supplies.
Ultralow 1.7µA Quiescent
Current in Light Load
When the output load is low, the
LT3971 and LT3991 decrease the
switching frequency to deliver power
to the output only when needed.
continued on page Sales Offices......................................40
L, Linear Express, Linear Technology, LT, LTC, LTM, BodeCAD, Burst Mode, FilterCAD, LTspice, OPTI-LOOP, Over-TheTop, PolyPhase, SwitcherCAD, µModule and the Linear logo are registered trademarks of Linear Technology Corporation.
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DESIGN FEATURES L
VIN
4.5V TO 38V
LT3971/91, continued from page Hook It Up and
Forget About It
The quiescent current of the LT3971 is
exceptionally low even when compared
to the self-discharge of a battery. Rechargeable batteries have significant
self-discharge. Nickel cadmium (NiCd)
batteries lose about 15% to 20% of
their charge in a month and nickel
metal hydride (NiMH) batteries are
even worse. There are several types
of low self-discharge NiMH batteries available, such as the SANYO
Eneloop, which lose about 15% to
30% of its charge per year. Lead acid
batteries discharge several percent
of their charge a month and lithium
secondary batteries discharge about
half as fast.
These discharge rates correspond
to over 100µA of self-discharge in the
worst case and tens of µA in the best
case. Primary batteries have much
Linear Technology Magazine • December 2009
OFF ON
VIN
EN
BOOST
0.47µF
PG
SS
4.7µF
LT3971
D1
BD
49.9k
SYNC
4.7µH
SW
RT
GND
VOUT
3.3V
1.2A
10pF
1.78M
FB
22µF
1M
D1: DIODES INC. DFLS240
4.0
3.5
INPUT CURRENT (µA)
Between current pulses, most of the
part’s internal circuitry is turned off,
to reduce the quiescent current to
only 1.7µA. Even with no load current, the feedback resistors and the
leakage of the Schottky catch diode
act as a load current of a few µA,
increasing the quiescent current of
the application circuit. By using a
few MΩ of feedback divider resistance
and a Schottky catch diode with low
leakage, only 2.8µA of input current
is consumed when regulating a 3.3V
output with no load from a 12V input.
The application in Figure 1 achieves
low input current over the entire input
voltage range when regulating a 3.3V
output with no load.
One way to demonstrate the low
current performance of the LT3971 is
to drive the part from a charged bulk
input capacitor. Using a 1000µF, 35V
electrolytic capacitor, with leakage less
than 1µA, charged to 16V, the LT3971
can regulate a 3.3V output with no load
for over an hour. The 1000µF capacitor drains at a rate of about 1V every
five minutes until the part drops out
at an input voltage of 4V. This type
of performance shows the LT3971’s
potential in energy harvesting systems
and back-up systems.
3.0
2.5
2.0
1.5
1.0
0
10
20
30
INPUT VOLTAGE (V)
40
Figure 1. The LT3971 can achieve ultralow input
current while regulating a 3.3V output with no load.
lower self-discharge rates. Alkaline
and lithium primaries can take up
to five to fifteen years to lose 20% of
their charge. This corresponds to only
a few micro-amps of self-discharge
current.
Compared to these numbers, the
LT3971’s quiescent current is over an
order of magnitude less than the selfdischarge of rechargeable batteries, so
the LT3971’s impact on battery life is
VSW
5V/DIV
IL
500mA/DIV
VOUT
20mV/DIV
5µs/DIV
VIN = 12V, VOUT = 3.3V
ILOAD = 10mA
COUT = 22µF
Figure 2. During light loads, the output voltage
ripple is controlled by single pulse burst mode.
For a 12VIN to 3.3VOUT application with 10mA
of load, the output voltage ripple is below
15mV with a 22µF output capacitor.
so small, you can hook it up and forget
about it. Primary batteries have selfdischarge comparable to the LT3971’s
quiescent current, so the battery only
drains about twice as fast as it would
if it was just sitting on a shelf.
Less Than 15mV of
Output Voltage Ripple
The output voltage ripple of the LT3971
is less than 15mV across the full load
range. During light load situations, the
regulator enters Burst Mode® operation where single current pulses are
used to recharge the output capacitor when the part detects the output
voltage has drooped below the regulation value. Single pulse operation is
critical to controlling output voltage
ripple, because multiple pulses would
quickly charge the output capacitor
excessively. The peak of each current
pulse is set to about 330mA, generating
consistent ripple performance across
the Burst Mode operation load range.
The switching waveforms in Figure
2 show the ripple performance for a
10mA load.
L DESIGN FEATURES
In typical hysteretic Burst Mode
implementations, the peak-to-peak
voltage of the output ripple is a fixed
value. In contrast, with the single pulse
Burst Mode implementation used in
the LT3971, the output ripple voltage
can be adjusted by changing the output capacitance. The peak current of
the pulses is independent of the output
capacitor size because a single, 330mA
pulse is always delivered. The total
charge delivered with each switching
pulse is constant, so the output voltage ripple in Burst Mode operation can
be reduced by increasing the output
capacitance. Figure 3 shows how the
output ripple in Burst Mode operation
decreases proportionally to increases
in the output capacitance. The peak
current in each switching pulse was set
so as to yield less than 15mV of ripple
even with a 22µF output capacitor.
Uncompromised
Fast Transient Response
and Full Feature Set
No compromises were made to achieve
the LT3971’s low quiescent current.
The part has good transient performance and a full feature set. The peak
current mode control scheme with
internal compensation maintains good
stability across load and temperature;
the user just has to include a 10pF
phase lead capacitor between the output and the FB pin. The response to
a 0.5A load step starting from both a
0.5A load and a 25mA load are shown
in Figure 4. The regulator displays
smooth transitions between Burst
VOUT
22µF OUTPUT CAP
2mV/DIV
VOUT
47µF OUTPUT CAP
2mV/DIV
VOUT
100µF OUTPUT CAP
2mV/DIV
10ms/DIV
VIN = 12V, VOUT = 3.3V
NO LOAD
Figure 3. Output voltage ripple decreases with increasing capacitor size in burst mode. The
output ripple is about 6mVP–P with a 22µF, 4mVP–P with a 47µF, and 2mVP–P with a 100µF output
capacitor. A 0.5 inch lead to a 1µF capacitor is used to help filter the ESL spike on the output
and care is taken to measure the ripple directly across the capacitor.
VOUT
100mV/
DIV
VOUT
100mV/DIV
IL
500mA/
DIV
IL
500mA/DIV
10µs/DIV
FIGURE 1 APPLICATION
VIN = 12V, VOUT = 3.3V
COUT = 47µF
10µs/DIV
FIGURE 1 APPLICATION
VIN = 12V, VOUT = 3.3V
COUT = 47µF
Figure 4. Transient responses for a 25mA to 525mA load step and a 0.5A to 1A load step. The
transition between burst mode and full frequency operation is smooth.
Mode operation and full switching
frequency.
The LT3971 switching frequency
can be programmed between 200kHz
and 2MHz with an external resistor.
By connecting an external clock to the
SYNC pin, the switching frequency
can be synchronized as fast as 2MHz.
A soft-start feature limits the inrush
current of the part by throttling the
switch current limit during start-up.
The SS pin is actively pulled down
R = 1k
+
–
11M
V
24V
+
VIN
EN
CBULK
100µF
1M
BOOST
0.47µF
PG
SS
4.7µF
LT3971
SW
D1
RT
BD
1nF
10pF
49.9k
SYNC
f = 800kHz
GND
* AVERAGE OUTPUT POWER CANNOT
EXCEED THAT WHICH CAN BE PROVIDED
BY HIGH IMPEDANCE SOURCE.
NAMELY,
V2
POUT(MAX)tη
4R
4.7µH
1M
FB
412k
VOUT
4V
1.2A*
100µF
WHERE V IS VOLTAGE OF SOURCE, R IS
INTERNAL SOURCE IMPEDANCE, AND η IS
LT3971 EFFICIENCY. MAXIMUM OUTPUT
CURRENT OF 1.2A CAN BE SUPPLIED FOR A
SHORT TIME BASED ON THE ENERGY
WHICH CAN BE SOURCED BY THE BULK
INPUT CAPACITANCE.
D1: DIODES INC. DFLS240
Figure 5. A LT3971 application circuit where the 1MΩ and 11MΩ resistor divider sets a 12V input voltage enable threshold to prevent the 24V, 1kΩ
impedance source from collapsing.
Linear Technology Magazine • December 2009
DESIGN FEATURES L
VOUT
200mV
/DIV
VIN
5V/DIV
VIN
1V/DIV
VOUT
2V/DIV
IL
500mA
/DIV
IL
1A/DIV
2ms/DIV
500µs/DIV
Figure 7. Even though the high impedance
source can not provide the power to supply
1.2A to the output, energy from the bulk input
capacitance can supply brief high current
output pulses.
Figure 6. As the output charges to 4V on startup, the 12V VIN(EN) threshold temporarily shuts
down the part to prevent the high impedance
input source from collapsing.
when the EN pin is low. Then a 1µA
current source into an external capacitor connected to the SS pin sets
the soft-start ramp rate when the part
starts-up. The LT3971 comes in a 10pin MSOP package or a 10-pin 3mm
× 3mm DFN package. Both package
types have an exposed pad that provides lower thermal resistance and a
ground connection.
between VIN and EN. When the input
voltage is greater than the VIN(EN)
threshold the LT3971 regulates the
output voltage, and when the input
voltage is below the VIN(EN) threshold
the part stops regulating the output
voltage.
High Impedance Input Source
A programmable input voltage enable
threshold is very useful when driving
the LT3971 with a high impedance
input source. These types of sources
could be distributed supplies, lines
used for both power and signaling, or
types of energy harvesting devices. A
buck regulator draws constant power
from the input, thus appearing to the
input as a negative impedance. When
the converter starts to draw current
from a high impedance source, the
voltage at the input pin starts to drop,
and the converter then draws even
more current. If the regulator draws
more power than the input supply can
provide, for example during start-up
when the output capacitor is being
Accurate Enable Pin
The quiescent current of the LT3971
is so low that in shutdown mode the
internal bandgap reference can still
operate, consuming only 700nA of
input current. This allows an accurate
1V enable pin threshold when VIN is
above 4.3V. When the enable pin is
above 1V, the part is enabled and can
switch, and when the enable pin is
below 1V, the part is shutdown and
cannot switch.
The accurate enable pin threshold
can be used to program an input
voltage enable threshold (VIN(EN)) by
connecting a simple resistor divider
charged, than the converter can
collapse the input supply. An input
voltage enable threshold solves this
problem by shutting the part down
when the input voltage collapses to
the VIN(EN) threshold. Figure 5 shows
an application where the LT3971 is
being driven by a 24V source with
a 1kΩ series resistance. The 1MΩ
and 11MΩ resistor divider sets a
12V VIN(EN) threshold on the input.
As the output capacitor charges to
its regulation value of 4V, the VIN(EN)
threshold prevents the input voltage
from collapsing below 12V, as seen
in Figure 6.
The output cannot, on average,
draw more power than the input can
supply with its high impedance. However, the LT3971 can source up to the
1.2A maximum output current for a
brief time, as long as the energy is supplied by the input capacitance. Figure
7 shows 1.2A of output current being
supplied for 2ms from the 100µF bulk
input capacitance. The ability of the
LT3971 to supply this type of pulsed
load is very important for satisfying
low duty cycle sensor applications
and energy harvesting applications,
which take advantage of both the low
quiescent current performance and
1.2A maximum load of the LT3971.
LT3991 48V to 3.3V
300kHz Application
The LT3991 has the same low quiescent
current performance and 1.2A maximum output current as the LT3971,
but can operate with input voltages
up to 55V. It also includes soft-start
continued on page VIN
4.3V TO 55V
250
VIN
EN
0.47µF
PG
SS
4.7µF
200
BOOST
LT3991
D1
RT
BD
162k
SYNC
GND
10µH
SW
VOUT
3.3V
1.2A
10pF
1M
D1: DIODES INC. DFLS260
150
100
tON(MIN)
50
1.78M
FB
SWITCH ON TIME (ns)
OFF ON
47µF
0
–55
–25
5
35
65
TEMPERATURE (°C)
95
125
Figure 8. The low minimum switch on time of the LT3991 allows the high step-down ratio, 48VIN to 3.3VOUT, at 300kHz switching frequency. This
yields a small solution size with a 10μH inductor and a 47μF ceramic output capacitor.
Linear Technology Magazine • December 2009
DESIGN FEATURES L
500nV/DIV
1s/DIV
AN124 F08
Figure 7. LTC6655 0.1Hz to 10Hz noise measures 775nV in 10-second sample time.
ground loop induced corruption. The
oscilloscope input signal is supplied
by an isolated probe; the sweep gate
output is interfaced with an isolation
pulse transformer. For more details,
see Linear Technology Application Note
124, Appendix C.
Noise Measurement
Circuit Performance
Circuit performance must be characterized prior to measuring LTC6655
noise. The preamplifier stage is verified
for >10Hz bandwidth by applying a
1µV step at its input (reference disconnected) and monitoring A2’s output.
Figure 3’s 10ms rise time indicates
35Hz response, insuring the entire
0.1Hz to 10Hz noise spectrum is supplied to the succeeding filter stage.
Figure 4 describes peak-to-peak
noise detector operation. Waveforms
include A3’s input noise signal (Trace
A), A7 (Trace B) positive/A8 (Trace C)
negative peak detector outputs and
LT3971/91, continued from page and external clock synchronization
features, and comes in a 10-pin MSOP
or 3mm × 3mm DFN package, both
with an exposed ground pad.
The LT3991 has a typical minimum
switch on time of 110ns at room and
150ns at 85°C, which allow higher
switching frequencies for large stepdown ratios when compared to other
parts with similar high input voltage
ratings. Figure 8 shows a 48V input
to a 3.3V output application with a
switching frequency of 300kHz. The
10µH inductor and 47µF output capacitor yield a small overall solution
Linear Technology Magazine • December 2009
DVM differential input (Trace D). Trace
E’s oscilloscope supplied reset pulse
has been lengthened for photographic
clarity.
Circuit noise floor is measured
by replacing the LTC6655 with a 3V
battery stack. Dielectric absorption
effects in the large input capacitor
require a 24-hour settling period before
measurement. Figure 5, taken at the
circuit’s oscilloscope output, shows
160nV 0.1Hz to 10Hz noise in a 10
second sample window. Because noise
adds in root-sum-square fashion,
this represents about a 2% error in
the LTC6655’s expected 775nV noise
figure. This term is accounted for by
placing Figure 2’s “root-sum-square
correction” switch in the appropriate position during reference testing.
The resultant 2% gain attenuation
first order corrects LTC6655 output
noise reading for the circuit’s 160nV
noise floor contribution. Figure 6, a
strip-chart recording of the peak-to-
size. The output capacitor can be a
small ceramic capacitor, as opposed
to a tantalum capacitor, because the
LT3991 does not need any output
capacitor ESR for stability.
Conclusion
The LT3971 and LT3991 are ultralow
quiescent current regulators that can
regulate a 12V input to a 3.3V output
during no load conditions with only
2.8µA of input current. Light load
operation with single current pulses
keeps the output voltage ripple to less
than 15mV. These buck regulators
can also provide up to 1.2A of output
peak noise detector output over six
minutes, shows less than 160nV test
circuit noise.4 Resets occur every 10
seconds. A 3V battery biases the input
capacitor, replacing the LTC6655 for
this test.
Figure 7 is LTC6655 noise after the
indicated 24-hour dielectric absorption soak time. Noise is within 775nV
peak-to-peak in this 10 second sample
window with the root-sum-square correction enabled. The verified, extremely
low circuit noise floor makes it highly
likely this data is valid. In closing, it
is worth mention that the approach
taken is applicable to measuring any
0.1Hz to 10Hz noise source, although
the root-sum-square error correction
coefficient should be re-established
for any given noise level. L
Notes
1The preamplifier structure must be carefully
prepared. See Appendix A in Linear Technology
Application Note 124, “Mechanical and Layout
Considerations,” for detail on preamplifier construction.
2Diode-connected JFETs’ superior leakage derives
from their extremely small area gate-channel junction. In general, JFETs leak a few picoamperes
(25°C) while common signal diodes (e.g. 1N4148)
are about 1,000× worse (units of nanoamperes at
25°C).
3Teflon and polystyrene dielectrics are even better
but the Real World intrudes. Teflon is expensive
and excessively large at 1µF. Analog types mourn
the imminent passing of the polystyrene era as the
sole manufacturer of polystyrene film has ceased
production.
4That’s right, a strip-chart recording. Stubborn, locally based aberrants persist in their use of such
archaic devices, forsaking more modern alternatives. Technical advantage could account for this
choice, although deeply seated cultural bias may
be indicated.
current. The LT3971 and LT3991 are
well suited for keep-alive and remote
monitoring systems with low duty
cycle, high current, pulsed outputs.
The wide input range from 4.3V up
to 38V for the LT3971, and 55V for
the LT3991, along with the programmable input voltage enable threshold
feature, allow these converters to be
driven from a wide range of input
sources. The ultralow quiescent current performance of the LT3971 and
LT3991 make them great choices for
battery-operated systems where power
conservation is critical. L
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