LTC1435AI datasheet(6/20 Pages) LINER

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LTC1435A

FUNCTIONAL DIAGRA

–

SWITCH

LOGIC

DROP

OUT

DET

0.6V

1.19V

INTVCC

–

180k

30k

–

SHUTDOWN

µA

1.19V

1.28V

–

4.8V

VIN

SHUTDOWN

1.19V

REF

CSS

RUN/SS

ITH

DFB*

SENSE+

SENSE–

EXTVCC

VOSENSE

µA

OSC

PGND

INTVCC

BOOST

SGND

VIN

SFB

COSC

RSENSE

VSEC

CSEC

VOUT

COUT

INTVCC

CIN

VIN

COSC

LDO

REG

RUN

SOFT

START

1435A • FD

VFB

gm = 1m

* FOLDBACK CURRENT LIMITING OPTION

(Refer to Functional Diagram)

OPERATION

Main Control Loop

The LTC1435A uses a constant frequency, current mode

step-down architecture. During normal operation, the top

MOSFET is turned on each cycle when the oscillator sets

the RS latch, and turned off when the main current com-

parator I1 resets the RS latch. The peak inductor current at

which I1 resets the RS latch is controlled by the voltage on

the ITH pin , which is the output of error amplifier EA. The

VOSENSEpin, described in the Pin Functions section, allows

EA to receive an output feedback voltage VFB from an ex-

ternal resistive divider. When the load current increases,

it causes a slight decrease in VFB relative to the 1.19V ref-

erence, which in turn causes the ITHvoltagetoincreaseuntil

the average inductor current matches the new load current.

While the top MOSFET is off, the bottom MOSFET is turned

on until either the inductor current starts to reverse, as

indicated by current comparator I2, or the beginning of the

next cycle.

The top MOSFET driver is biased from floating bootstrap

capacitor CB, which normally is recharged during each off

cycle. However, when VIN decreases to a voltage close to

VOUT, the loop may enter dropout and attempt to turn on

the top MOSFET continuously. The dropout detector counts

the number of oscillator cycles that the top MOSFET remains

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