LT1577CS-3.3/2.8#TRPBF【PDF 13/20 页】IC REG CTRLR DUAL OUT 16SOIC

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LT1575/LT1577

APPLICATIONS INFORMATION

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

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. Todays 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

SHDN

GND

OUT

IPOS

INEG

GATE

COMP

1礔

220礔

GND

1575/77 TA11

OUT

3.5V

R3*

0.007&

7.5k

12V

LT1575-3.5

1000pF

1礔

RESET

R3 IS MADE FROM

FREE PC BOARD

TRACE

C6 = 24 ?1礔 X7R

CERAMIC SURFACE

MOUNT CAPACITORS.

PLACE C6 IN THE

MICROPROCESSOR

SOCKET CAVITY

VN2222L

IRFZ24

10pF

C6**

24礔

TYPICAL APPLICATIONS N

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