LT3752 datasheet(19/52 Pages) AD

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LT3752/LT3752-1

Rev. C

For more information www.analog.com

OPERATION

Introduction

The LT3752/LT3752-1 are primary side, current mode,

PWM controllers optimized for use in a synchronous

forward converter with active clamp reset. Combined with

an integrated housekeeping controller, each IC provides

a compact, versatile, and highly efficient solution. The

LT3752 allows VIN pin operation between 6.5V and 100V.

For applications with system input voltages greater than

100V, the LT3752-1 allows RC start-up from input voltage

levels limited only by external components. The LT3752

and LT3752-1 based forward converters are targeted for

power levels up to 400W and are not intended for battery

chargerapplications.Forhigherpowerlevelstheconverter

outputs can be stacked in series. Connecting UVLO_VSEC

pins, OVLO pins, SS1 pins and SS2 pins together allows

blocks to react simultaneously to all fault modes and

conditions.

Each IC contains an accurate programmable volt-second

clamp. When set above the natural duty cycle of the con-

verter, it provides a duty cycle guardrail to limit primary

switch reset voltage and prevent transformer saturation

during load transients. The accuracy and excellent line

regulation of the volt-second clamp provides VOUT regu-

lation for open-loop conditions such as no opto-coupler,

reference or error amplifier on the secondary side.

For applications not requiring isolation but requiring high

step-down ratios, each IC contains a voltage error ampli-

fier to allow a very simple nonisolated, fully regulated

synchronous forward converter.

The integrated housekeeping controller reduces the com-

plexity and size of the main power transformer by avoid-

ing the need for extra windings to create bias supplies.

Secondary side ICs no longer require start-up circuitry

and can operate even when output voltage is 0V.

A range of protection features include programmable

overcurrent (OC) hiccup mode, programmable system

input undervoltage lockout (UVLO), programmable

system input overvoltage lockout (OVLO) and built-in

thermal shutdown. Programmable slope compensation

and switching frequency allow the use of a wide range of

output inductor values and transformer sizes.

LT3752 Part Start-Up

LT3752 start-up is best described by referring to the Block

Diagram and to the start-up waveforms in Figure 3. For

part start-up, system input voltage must be high enough

to drive the UVLO_VSEC pin above 1.25V and the VIN pin

must be greater than 6.5V. An internal linear regulator is

activated and provides a 7V INTVCC supply for all gate

drivers. The housekeeping controller starts up before the

forward controller. An internal soft-start (HSS) ramps

the housekeeping HCOMP pin to allow switching at the

gate driver output HOUT to drive an external N-channel

MOSFET. The housekeeping controller output voltage

VHK is regulated when the HFB pin reaches 1.0V. VHK can

be used to override INTVCC to reduce power in the part,

increase efficiency and to optimize the INTVCC level. Dur-

ing start-up the housekeeping controller switches at the

programmed switching frequency (fOSC) folded back by

1/4.6. The SS1 pin of the forward controller is allowed to

start charging when VHK reaches 96% of its target value

(PGOOD). When SS1 reaches 1.25V, the SS2 pin begins

to charge, controlling COMP pin rise and the soft-start of

output inductor peak current. The SS1 pin independently

soft starts switching frequency and a volt-second clamp.

As SS1 charges towards 2.6V the switching frequencies

of both controllers remain equal, synchronized and soft

started towards full-scale fOSC.

If secondary side control already exists for soft starting

the converter output voltage then the SS2 pin can still be

used to control initial inductor peak current rise. Simply

programming the primary side SS2 soft-start faster than

the secondary side allows the secondary side to take over.

If SS2 is not needed for soft-start control, its pull-down

strength and voltage rating also allow it to drive the input

of an opto-coupler connected to INTVCC. This allows

the option of communicating to the secondary side that

switching has begun.

LT3752-1 Part Start-Up

TheLT3752-1start-upofhousekeepingsupplyandforward

converter are similar to the LT3752 except for a small

change in architecture and VINpinlevel.LT3752-1start-up

is best described by referring to the Block Diagram and to

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