13
LT1575/LT1577
APPLICATIONS INFORMATION
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very large current spike as soon as the 5V supply started
to ramp up. However, undervoltage lockout circuit COMP2,
which monitors the IPOS supply voltage, holds Q3 on and
pulls the COMP pin low until the IPOS voltage increases
to greater than the internal 1.21 reference voltage. The
undervoltage lockout circuit then smoothly releases the
COMP pin and allows the output voltage to come up in
dropout from the input supply voltage. An additional
benefit derived from the speed of the LT1575 feedback
loop is that turn-on overshoot is virtually nonexistent in
a properly compensated system.
An additional circuit feature is built-in to the LT1575 fixed
voltage versions. When the regulator circuit starts up, it
must charge up the output capacitors. The output voltage
typically tracks the input voltage supply as it ramps up with
the difference in input/output voltage defined by the drop-
out voltage. Until the feedback loop comes into regulation,
the circuit operation results in the GATE pin being at the
positive V
IN
rail, which starts the timer at the SHDN pin if
the current limit amplifier is disabled. However, internal
comparator COMP4 monitors the input/output voltage
differential. This comparator does not permit the shut-
down timer to start until the differential voltage is greater
than 500mV. This permits normal start-up to occur.
One final benefit is derived in using an LT1575 fixed
voltage version. Today’s highest performance micropro-
cessors dictate that precision resistors must be used with
currently available adjustable voltage regulators to meet
the initial set point tolerance. The LT1575 fixed voltage
versions incorporate the precision resistor divider into the
IC and still maintain a 1% output voltage tolerance over
temperature. Thus, the LT1575 fixed voltage versions
completely eliminate the requirement for precision resis-
tors and this results in additional system cost savings.
Applications Support
Linear Technology invests an enormous amount of time,
resources and technical expertise in understanding, de-
signing and evaluating microprocessor power supply so-
lutions for system designers. As processor speeds and
power increase, the power supply challenges presented to
the motherboard designer increase as well. Application
Note 69, “Using the LT1575 Linear Regulator Controller,”
has been written and serves as an extremely useful guide
for this new family of ICs. This Application Note covers
topics including PC board layout for the LT1575/LT1577
family, MOSFET selection criteria, external component
selection (capacitors) and loop compensation. Linear
Technology welcomes the opportunity to discuss, design,
evaluate and optimize a microprocessor power supply
solution with a customer. For additional information,
consult the factory.
UltraFast Transient Response 5V to 3.5V Low Dropout Regulator
with Current Limit and Timer Latchoff
1
2
3
4
8
7
6
5
SHDN
V
IN
GND
OUT
IPOS
INEG
GATE
COMP
C2
1µF
C5
220µF
5V
GND
1575/77 TA11
V
OUT
3.5V
5A
R2
5Ω
R3*
0.007Ω
R1
7.5k
12V
LT1575-3.5
C4
1000pF
C1
1µF
RESET
R3 IS MADE FROM
“FREE” PC BOARD
TRACE
C6 = 24 × 1µF X7R
CERAMIC SURFACE
MOUNT CAPACITORS.
PLACE C6 IN THE
MICROPROCESSOR
SOCKET CAVITY
*
**
Q2
VN2222L
Q1
IRFZ24
+
C3
10pF
C6**
24µF
TYPICAL APPLICATIONS N
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