LTC1433 datasheet(6/20 Pages) LINER

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LTC1433/LTC1434

FUNCTIONAL DIAGRA

OPERATION (Refer to Functional Diagram)

Main Control Loop

The LTC1433/LTC1434 is a constant frequency, pulse-

width modulated current mode switching regulator. Dur-

ing normal operation, the internal P-channel power MOSFET

is turned on each cycle when the oscillator sets the RS

latch FF3, and turned off when the main current compara-

tor ICOMP resets the latch. The peak inductor current at

which the ICOMP resets the RS latch is controlled by the

voltage on the ITHpin , which is the output of error amplifier

GM. Pins VPROG and VOSENSE, described in the Pin Func-

tions section, allow GM to receive an output feedback

voltage VFB from either the internal or external resistive

dividers. When the load current increases, it causes a

slight decrease in VFB relative to the 1.19V reference,

which in turn causes the ITH voltage to increase until the

average inductor current matches the new load current.

The main control loop is shut down by pulling the RUN/SS

pin low. Releasing RUN/SS allows an internal 3

µA current

source to charge up the soft start capacitor CSS. When CSS

reaches 1.3V, the main control loop is enabled with the ITH

voltage clamped at approximately 30% of its maximum

value. As CSS continues to charge, ITH is gradually re-

leased allowing normal operation to resume.

Comparator OVDET guards against transient overshoots

> 7.5% by turning off the P-channel power MOSFETs and

keeping them off until the fault is removed.

Low Current Operation

The LTC1433/LTC1434 have two internal P-channel

MOSFETs sized for low and high load current conditions.

At low load current, only the small MOSFET will be turned

on while at high load current both MOSFETs will be on.

–

LOBAT

–

LIDET

OVDET

POR

BSW

PWRVIN

SSW

ITH

LBI

LBO

VREF

PLL LPF

PLLIN

COSC

180k

0.6V

SVIN

30k

VREF + 89mV

VSET

IPEAK

DET

ICOMP

12mV

RUN/SS

50k

120k

REF

AND

VCC

SHDN

VREF (1.19V)

VREF

PGND

SGND

VCC

SVIN

FF3

FF2

FF1

POR

PLL

SHDN

FREQ

SHIFT

OSC

VCO

SLOPE

COMP

RSENSE

0.143

Ω

VOLTAGE

SELECT

60k

240k

60k

VPROG

VOSENSE

1433/34 FD

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