Dec 2005 Low Ripple Micropower SOT-23 Buck Regulator with Integrated Boost and Catch Diodes Accepts Inputs to 40V

DESIGN IDEAS
Low Ripple Micropower SOT-23 Buck
Regulator with Integrated Boost and
Catch Diodes Accepts Inputs to 40V
by Leonard Shtargot
Introduction
The LT3470 is a micropower buck
regulator that integrates a 300mA
power switch, catch diode and boost
diode into a low profile 8-Pin ThinSOT
package (see Figure 1). The combination of single cycle Burst Mode and
continuous operation allows the use
of tiny inductor and capacitors while
providing a low ripple output to loads
of up to 200mA. With its wide input
range of 4V to 40V and low quiescent
current of 26µA (12V in to 3.3V out) the
LT3470 can regulate a wide variety of
power sources, from 2-cell Li-Ion batteries to unregulated wall transformers
and lead acid batteries.
3
BIAS
7
+
–
BOOST
500ns
ONE SHOT
R
Q′
S
Q
SW
–
ENABLE
5V, 200mA from 40V
Consumes Less than
1mW at No Load
SHDN
VREF
1.25V
6
5
+
BURST MODE
DETECT
2 NC
1
Figure 2 shows a 5V, 200mA supply
that accepts inputs from 5.5V to 40V.
While the output is in regulation and
with no load the power loss is lower
than 1mW. The LT3470 can also be
put in a shutdown mode that reduces
the input current to <1µA by pulling
the SHDN pin low. When always-on
operation is desired, the SHDN pin
can be tied to VIN.
The LT3470 uses a control system
that offers low (<10mV) ripple at the
VIN
gm
FB
GND
8
4
Figure 1. The block diagram of the LT3470 shows the integrated boost and catch Schottky
diodes. Inductor current is kept under control at all times by monitoring the VIN current as
well as the catch diode current, thereby providing short circuit protection even if VIN = 40V.
output while keeping quiescent current to a minimum. When output load
is light, the LT3470 remains in sleep
mode while periodically waking up
for single switch cycles to keep the
output in regulation. The current limit
of these single switch cycles is about
100mA, which keeps output ripple to
a minimum. At greater output loads
the LT3470 no longer enters sleep
mode, and instead servos the peak
switch current limit (up to 300mA) to
regulate the output. See Figure 3 for
operating waveforms.
continued on page 36
90
1000
VIN = 12V
80
BOOST
LT3470
OFF ON
SHDN
0.22µF
33µH
SW
BIAS
22pF
2.2µF
VOUT
5V
200mA
GND
FB
604k
1%
200k
1%
22µF
100
60
50
10
40
30
POWER LOSS (mW)
VIN
70
EFFICIENCY (%)
VIN
5.5V TO 40V
1
20
10
0.1
1
10
LOAD CURRENT (mA)
100
0.1
Figure 2. The LT3470 uses a minimum of board space and external components while delivering wide onput range and high efficiency.
This buck regulator supplies up to 200mA at 5V from inputs up to 40V. Input power loss is below 1mW when there is no output load.
34
Linear Technology Magazine • December 2005
DESIGN IDEAS
0.1µF
LTC3035
INTPUT
AC
20mV/DIV
DUAL
ALKALINE
BATTERY
1µF
1µF
LTC3035
OFF ON
LTC3035
OUTPUT
AC
20mV/DIV
CM
BIAS
CP
IN
SHDN
OUT
140k
1µF
VOUT = 1.8V
IOUT ≤ 300mA
ADJ
GND
40.2k
20µs/DIV
Figure 3. Input and output waveforms to
the LTC3035 in the Li-Ion to 3.3V application,
showing its excellent ripple rejection (IOUT =
25mA, LTC3440 in Burst Mode®)
High Efficiency,
Low Noise Li-Ion to 3.3V
Figure 2 shows a high efficiency and
low noise lithium-ion to 3.3V solution.
The LTC3440, a buck-boost converter,
converts the Li-Ion battery voltage to an
efficient intermediate voltage (3.4V) at
the input of the VLDO. The LTC3035
then regulates this intermediate voltage down to 3.3V, providing a lower
noise output voltage. Figure 3 shows
the input and output waveforms of the
LTC3035 at 25mA of output current,
illustrating its excellent power supply
rejection characteristics for a lower
noise solution.
For optimum total efficiency, the
input to output voltage differential
across the LDO should be as small as
possible, since the magnitude of the
dissipated power equals the product of
the voltage differential and the output
current. Because of the LTC3035’s
LT3470, continued from page 34
The fast cycle-by-cycle current
limit of the LT3470 keeps the switch
and inductor currents under control
at all times. In addition, the LT3470
uses hysteretic mode control where
the switching frequency automatically
adjusts to accommodate variations in
Figure 4. A very low dropout dual-alkaline to 1.8V application
very low dropout voltage, its input
voltage can be programmed to only
100mV above the 3.3V output and still
maintain regulation at 300mA. Conventional LDOs with higher dropout
voltages force greater input and output
voltage differentials, effectively reducing efficiency by the same ratio.
Double Alkaline to 1.8V LDO
Handheld applications using two alkaline batteries in series demand low
power solutions that use as much of
the battery’s operating voltage range
as possible. In Figure 4, two series
alkaline batteries are regulated down
to provide a 1.8V supply taking advantage of the LTC3035’s excellent
dropout characteristics.
The dropout voltage and maximum
output current capabilities of typical
low power LDOs using P-type transistors suffer as the input voltage supply
decreases, since the power transistor’s
overdrive reduces. With input and
VIN and VOUT. This means that the part
switches at a slower frequency when
the output is in short circuit or when
VIN/VOUT ratio is high. This ensures
that the LT3470 can handle a short
circuit at the output even if VIN = 40V
and the inductor value is small. It
NO LOAD
10mA LOAD
VOUT
20mV/DIV
VOUT
20mV/DIV
IL
100mA/DIV
IL
100mA/DIV
1ms/DIV
5µs/DIV
Figure 3. Operating waveforms show the output voltage ripple remains at 10mV
in BurstMode operation, while requiring only a 22µF ceramic output capacitor.
36
output voltages near 1.8V, conventional low power LDOs may have
dropout voltages over 200mV, if they
can deliver 300mA of output current
at all. Using the LTC3035, the battery
voltage can discharge much further
to only about 50mV above the 1.8V
output before the LDO begins to drop
out at 300mA. Allowing the battery to
discharge longer essentially extends
the battery life for the application
when compared to solutions that use
higher dropout LDOs.
Conclusion
The very low dropout characteristics
of the LTC3035 can be exploited in
battery-powered applications to obtain
higher efficiency and increased battery life. Its very low dropout voltage,
excellent power supply rejection, lowquiescent current, and small solution
size make the LTC3035 an ideal choice
for many low power, handheld battery
applications.
is, however, important to choose an
inductor that does not saturate excessively at currents below 400mA to
guarantee short circuit protection.
Conclusion
The LT3470 is a small buck regulator with a unique combination of
features that make it a great choice
in applications requiring small size,
high efficiency across a wide range of
currents, and low output ripple. It can
deliver up to 200mA from inputs as
high as 40V using only an inductor,
four small ceramic capacitors, and two
resistors while consuming only 26µA
during no load operation.
Linear Technology Magazine • December 2005
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