Dec 2005 Taking Full Advantage of Very Low Dropout Linear Regulators

DESIGN IDEAS
Taking Full Advantage of Very Low
Dropout Linear Regulators by Joe Panganiban
Introduction
70
60
DROPOUT VOLTAGE (mV)
Linear regulators are generally considered inefficient step-down DC/DC
converters, but low dropout linear
regulators (LDOs) can be a good fit in
many handheld battery applications
where low power and efficient power
conversion are critical. The lower the
dropout voltage, the more efficient the
LDO solution. Generally, LDOs with
a very small dropout voltage come at
the expense of increased package size
and higher quiescent current. The
LTC3035 overcomes these tradeoffs
by offering a very low dropout voltage
without sacrificing small solution size
or low power.
The LTC3035 is a micropower,
VLDO™ (very low dropout) linear
regulator, which operates from input
voltages between 1.7V and 5.5V. The
device is capable of supplying 300mA
of continuous output current with an
ultra-low dropout voltage of 45mV
typical (see Figure 1). The output
voltage is externally adjustable over a
wide voltage range, spanning between
0.4V and 3.6V.
The LTC3035 is ideal for batterypowered applications where low power,
low dropout, low noise, and small
solution size are essential. Under noload conditions, the chip draws only
50
TA = 125°C
40
TA = 25°C
30
20
TA = –40°C
10
0
0
50
100
200
150
IOUT (mA)
250
300
Figure 1. Typical dropout
voltage versus load current
100µA from the VIN supply, and drops
to 1µA when in shutdown. The LDO is
stable for all ceramic capacitors down
to 1µF. Other features include output
short-circuit protection, reverse output current protection, and thermal
overload protection, all available in a
tiny 3mm × 2mm DFN package.
Low Dropout from an
NMOS Pass Device
Conventional LDOs integrate a P-type
transistor (either PNP or PMOS) as the
power pass device to deliver current
from the input supply to its output.
The LTC3035, instead, incorporates an
NMOS transistor as its pass element
in a source-follower configuration.
This architecture allows for several
performance advantages over conventional P-type LDOs, such as greater VIN
power supply rejection, lower dropout
voltage, and better transient response
characteristics, while maintaining a
smaller solution size.
Using an NMOS pass device is
not entirely transparent. In order
to achieve low dropout performance
using an NMOS pass device, the LDO
circuitry must be capable of driving the
NMOS gate above the VIN supply. This
implies that a separate higher voltage
supply is necessary to power the LDO
circuitry. For many applications, the
luxury of an extra higher supply is
simply unavailable. The LTC3035
overcomes this problem by including a
built-in charge pump that generates a
higher BIAS supply from the VIN input
to power its LDO circuitry. The charge
pump requires only a 0.1µF flying capacitor and a 1µF bypass capacitor for
operation. The value of the generated
BIAS supply is adaptively controlled to
provide sufficient gate drive over the
full VIN operating range, optimizing
the current carrying capabilities and
dropout characteristics of the VLDO
regulator.
0.1µF
10µH
S
S
3
VIN = 2.7V TO 4.2V
LTC3440
7
S
VIN
8
Li-Ion
+
10µF
2
*
1
RT
60.4k
S
OFF ON
SW2
SW1
VOUT
SHDN/SS
FB
MODE/SYNC
VC
RT
GND
S
3.4V
600mA
6
CM
CP
1µF
BIAS
IN
1µF
4
LTC3035
SHDN
OUT
S
GND
ADJ
S
S
22µF
S
15k
5
200k
S
1µF
VOUT = 3.3V
IOUT ≤ 300mA
40.2k
1.5nF
10
S
294k
357k
9
S
S
S
S
*1 = Burst Mode OPERATION
0 = FIXED FREQUENCY
C1: TAIYO YUDEN JMK212BJ106MG
C2: TAIYO YUDEN JMK325BJ226MM
L1: SUMIDA CDRH6D38-100
Figure 2. A high-efficiency and low-noise lithium-ion to 3.3V solution
Linear Technology Magazine • December 2005
35
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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