November 2004 - Power Supply Tracking for Linear Regulators

DESIGN FEATURES
Power Supply Tracking for
Linear Regulators
Introduction
(see “Versatile Power Supply Tracking
without MOSFETs” from Linear Technology Magazine, February, 2004 ) but
it is easily adapted to linear regulators,
including popular low-dropout (LDO)
types. Summarized here are several
techniques for controlling linear regulators with the LTC2923.
The LTC2923 provides simple and
versatile control over the power-up
and power-down behavior of switching
power supplies. It allows several supplies to track the voltage of a master
supply, so that their relative voltages
meet the stringent specifications for
the power up of modern digital
semiconductors, such as DSPs, microprocessors, FPGAs and ASICs. The
LTC2923 is specifically designed to
work with switching power supplies
Monolithic Regulators
Table 1 lists three popular monolithic
linear regulators that have been tested
with the LTC2923. Using these three
Table 1. New monolithic linear regulators
Regulator
IOUT(MAX) (V)
VIN(MIN) (V)
VIN(MAX) (V)
VDROPOUT (V)
LT3020
100mA
0.9
10
0.15
LTC1844
150mA
1.6
6.5
0.11
LTC3025
300mA
0.9
5.5
0.045
3.3V
IN
2.2µF
3.3V
CGATE
0.1µF
0.1µF
VCC GATE
OFF ON
ON
SHDN
ADJ
GND
20k
2.2µF
FB1
3.3V
1µF
RAMPBUF
232k
SDO
TRACK1
IN
OUT
BIAS LTC3025
LTC3025
SHDN
107k
VOUT = 1.5V
107k
1µF
ADJ
GND
39.2k
FB2
TRACK2
124k
VOUT = 2.5V
232k
RAMP
LTC2923
107k
OUT
LT3020-ADJ
GND
Figure 1. An LTC2923 causes the outputs of the LT3020
and LTC3025 to track during power-up and power-down.
by Dan Eddleman
monolithic LDOs with the LTC2923 is
generally very simple:
❑ The LTC3020 is a 100mA low
dropout regulator (LDO) that operates with input supply voltages
between 1V and 10V. Since its
ADJ pin behaves like the feedback pin on most switching regulators, tracking the LTC3020’s
output using the LTC2923 is
simple. The standard circuits and
design procedures shown in the
LTC2923 data sheet require no
modification when used with the
LTC3020 (Figures 1 and 2).
❑ The LTC3025 is a 300mA monolithic CMOS LDO that regulates
input supplies between 0.9V and
5.5V, while a bias supply between 2.5V and 5.5V powers the
part. Similar to the LT3020, the
LTC3025’s ADJ pin is operationally identical to common switchers. For that reason, the LTC3025
combined with an LTC2923
provides a simple supply tracking solution for loads less than
300mA (Figures 1 and 2).
❑ The LTC1844 CMOS LDO drives
loads up to 150mA with input
supply voltages between 1.6V and
6.5V. When used in conjunction
with the LTC2923, a feedforward
capacitor should be included as
described in the “Adjustable Operation” section of the LTC1844
data sheet. Otherwise, no special
considerations are necessary.
The LTC1761 Family of
Monolithic, Bipolar Regulators
2.5V LT3020 OUT
1.5V LTC3025 OUT
1V/DIV
10ms/DIV
10ms/DIV
Figure 2. The outputs of the LT3020 and LTC3025 low-dropout linear regulators
ramp-up and ramp-down together. (Output of circuit in Figure 1.)
14
Table 2 shows the LTC1761 family
of monolithic, bipolar low dropout
regulators. These regulators cover a
wide range of load currents and offer
outstanding transient response and
low noise, making them a popular
choice for applications with loads less
than 3A.
In these regulators, the ADJ pin
draws excess current when the OUT
Linear Technology Magazine • November 2004
DESIGN FEATURES
VIN
IN
OUT
LT1761
3.3V MASTER
2.5V LT1761 OUT
1V/DIV
LT1761 HOLDS
AT 1V
GND
VOUT
ADJ
R2
xxLTC2923
FBx
R1
SHDN ASSERTED
SHDN RELEASED
10ms/DIV
10ms/DIV
Figure 3. LT1761/LT1962/LT1762/LT1763/LT1963A/LT1764A with adjustable outputs only
track above 1V unless modified as discussed in this article. The SHDN pin of the LDO is active
before the ramp-up and after ramp-down.
pin drops below about 1V, a region of
operation that LDOs do not normally
experience. Nevertheless, an LDO
which tracks another supply, enters
this region when the output tracks
below 1V (Figure 3). If this excess current is not accounted for, the output
of the LDO will be slightly higher than
ideal when it tracks below 1V. Three
techniques have been used to successfully track outputs of this LDO
family below 1V.
If low dropout voltages are not
necessary, simply connect two diodes
in series with the OUT pin (Figure
4). In this configuration, the OUT
pin remains two diode drops above
the circuit’s output. As a result, the
LDO remains in its normal region of
operation even when the output is
driven near ground. Since the feedback
resistors are connected to the output,
the LDO regulates the voltage at the
circuit output instead of the LDO’s
OUT pin. Diode voltage varies with
both load current and temperature, so
verify that the output is low enough at
the minimum diode voltage. Likewise,
the input voltage must be high enough
to regulate the output when the diode
drops are at their maximum. This solution effectively increases the dropout
voltage of the linear regulator by two
diode drops. Therefore, applications
that require a low dropout voltage
are better served by the solutions
that follow.
Consider using the LTC1761,
LT1962, LT1762, or LT1763 voltage
regulators when the load is less than
500mA and a low dropout voltage is
necessary. A fixed output part, (such
as the LTC1763A-1.5) can be used
as an adjustable LDO if the SENSE
pin is treated like an ADJ pin with a
feedback voltage of 1.5V (Figure 5).
The SENSE pin on the fixed output
parts draws about 10µA regardless
of the OUT pin’s voltage, unlike the
ADJ pin on the adjustable parts. When
choosing feedback resistors, minimize
the output error by compensating for
the extra 10µA of current that appears
across the upper resistor. Also, use
small valued resistors to minimize the
error due to the 0µA to 20µA data sheet
limits while avoiding values that are
so small that the LTC2923’s 1mA IFB
will be unable to drive the output to
ground. To satisfy these constraints,
Figure 4. Diodes placed in series with the OUT
pin allow the LT1761 to track down to 0V.
ensure that the parallel combination
of the two feedback resistors is slightly
greater than 1.5kΩ. For most output
voltages, this reduces the output error due to the SENSE pin current to
about 1%.
For applications that require higher
load currents and a low dropout voltage, the LT1963A and LT1764A may be
appropriate. These parts are specified
for 1.5A and 3A load currents respectively. Unfortunately, the SENSE pins
on these fixed output parts draw about
600µA.
To use these parts, configure an
operational amplifier to buffer the
voltage from the feedback resistors
to the SENSE pin of the 1.5V fixed
output versions (Figure 6). If the op
amp is configured with a voltage gain
of 2, the 1.5V regulator in combination with the op amp behaves as an
adjustable output regulator with a
0.75V reference voltage. The input
to the op amp now serves as the
ADJ input of the new regulator. This
technique allows the use of the high
current LT1963A/LT1764A where the
voltage loss of series diodes would be
unacceptable. It also works for the
LT1761, LT1962, LT1762, and LT1763
in cases where the 10µA ADJ pin curcontinued on page 35
Table 2. LT1761 family of low-dropout linear regulators
Regulator
IOUT(MAX) (V)
VIN(MIN) (V)
VIN(MAX) (V)
VDROPOUT (V)
LT1761
100mA
1.8
20
0.30
LT1762
150mA
1.8
20
0.30
LT1962
300mA
1.8
20
0.27
LT1763
500mA
1.8
20
0.30
LT1963A
1.5A
2.1
20
0.34
LT1764A
3A
2.7
20
0.34
Linear Technology Magazine • November 2004
VIN
IN
VOUT
OUT
LT1763-1.5
1.5V
SENSE
10µA
GND
R2
LTC2923
FBx
R1
Figure 5. The fixed-output LT1763-1.5 can
track down to 0V, has low dropout, and a
resistive divider can be used for outputs
greater than 1.5V.
15
DESIGN IDEAS
Optimizing for Efficiency
While the LT3461A (boost) and
LT3462A (inverting) are optimized
for small size, the LT3461 (boost) and
LT3462 (inverting) are intended for applications requiring higher efficiencies
or high conversion ratios. The lower
switching frequencies translate to
higher efficiencies because of a reduction in switching losses.
The LT3461 (boost) is guaranteed to
a maximum switch duty cycle of 92%
in continuous conduction mode, and
the LT3462 (inverting) is guaranteed to
a maximum switch duty cycle of 90%,
which enables high conversion ratios
at relatively high output currents.
LTC2923, continued from page 15
rent produces an unacceptable output
voltage error.
Drivers for External,
High Current Pass Devices
Table 3 summarizes the characteristics of the LT1575 and LT3150 low
dropout regulators. These devices
drive external N-channel MOSFET
pass devices for high current/high
power applications. The LTC3150
Although high conversion ratios can
also be obtained using discontinuous conduction mode (DCM)—where
current in the inductor is allowed to
go to zero each cycle—the DCM technique requires higher switch currents
and larger inductors/rectifiers than
a system operating in continuous
conduction mode at the same load current. Because the LT3461 can switch
at 1.3MHz in continuous conduction
mode with up to 92% switch duty cycle,
and the LT3462 at 1.2Mhz, 90% duty,
they are the most compact solutions
available for outputs 5 to 10 times
the supply voltage. For example, the
LCD bias circuit of Figure 7 provides
additionally includes a boost regulator that generates gate drive for the
external FET.
The LTC2923 tracks the outputs of
the LT1575 and LT3150 without any
special modifications. Because these
linear regulators only pull the FET’s
gate down to about 2.6V, low-threshold
FETs may not allow the output to fall
below a few hundred millivolts. This is
acceptable for most applications.
18mA at 25V from a 3.3V supply and
occupies as little as 50mm2 of board
space. Figure 8 shows that the efficiency of the 25V converter is quite
good, peaking at 79% for a 4.2V supply. Figure 9 shows a 3.3V to –25V,
14mA inverter with efficiency above
70% (Figure 10).
Conclusion
The LT3461, LT3461A, LT3462 and
LT3462A provide very compact boost
and inverter solutions for a wide
input voltage range of 2.5V to 16V,
and outputs to ±38V, making these
devices a good fit in a variety of applications.
VIN
IN
OUT
VOUT
LT1963-1.5
SENSE
GND
VIN
1.5V
R
LTC2923
FBx
R2
+
0.75V
LT1006
–
R1
R
Table 3. Drivers for external, high current pass devices
Regulator
IOUT(MAX) (V)
VIN(MIN) (V)
VIN(MAX) (V)
VDROPOUT (V)
LT3150
10A*
1.4
10
0.13
LT1575
*
N/A
22
*
*Depends on selection of external MOSFET
LT1990/91/95, continued from page 4
operating-point—and resistors to set
gain. High quality resistors consume
precious printed circuit board real
estate, and test time. In contrast, the
LT1995 provides on-chip resistors
for voltage division and gain setting
in a highly integrated video-speed op
amp.
Figure 5 shows a simple way to drive
AC-coupled composite video signals
over 75Ω coaxial cable using minimum
component count. In this circuit, the
input resistors form a supply splitter
Linear Technology Magazine • November 2004
for biasing and a net attenuation of
0.75. The feedback configuration provides an AC-coupled gain of 2.66, so
that the overall gain of the stage is 2.0.
The output is AC-coupled and series
back-terminated with 75Ω to provide a
match into terminated video cable and
an overall unity gain from signal input
to the destination load. An output
shunt resistor (10kΩ in this example)
is always good practice in AC-coupled
circuits to assure nominal biasing of
the output coupling capacitor.
Figure 6. Using an op amp with the LT1963-1.5
allows lower output voltages and removes error
due to the SENSE pin current.
Authors can be contacted
at (408) 432-1900
Full Bridge Load Current Monitor
Many new motor-drive circuits employ
an H-bridge transistor configuration
to provide bidirectional control from
a single-voltage supply. The difficulty
with this topology is that both motor leads “fly,” so current sensing
becomes problematic. The LT1990
offers a simple solution to the problem
by providing an integrated difference
amp structure with an unusually high
common-mode voltage rating, up to
±250VDC.
35