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LT1425

TI I G DIAGRA

VSW

VOLTAGE

VIN

GND

OFF

MINIMUM tON

ENABLE DELAY

MINIMUM ENABLE TIME

1425 TD

OFF

SWITCH

STATE

FLYBACK AMP

STATE

0.80

VFLBK

COLLAPSE

DETECT

ENABLED

DISABLED

OPERATION

The LT1425 is a current mode switching regulator IC that

has been designed specifically for the isolated flyback

topology. The special problem normally encountered in

such circuits is that information relating to the output

voltage on the isolated secondary side of the transformer

must be communicated to the primary side in order to

maintain regulation. Historically, this has been done with

optoisolators or extra transformer windings. Optoisolator

circuits waste output power and the extra components

they require increase the cost and physical volume of the

power supply. Optoisolators can also exhibit trouble due

to limited dynamic response (temporal), nonlinearity,

unit-to-unit variation and aging over life. Circuits

employing extra transformer windings also exhibit defi-

ciencies. The extra winding adds to the transformer’s

physical size and cost. Dynamic response is often

mediocre. There is usually no method for maintaining

load regulation versus load.

The LT1425 derives its information about the isolated

output voltage by examining the primary side flyback

pulse waveform. In this manner no optoisolator nor extra

transformer winding is required. This IC is a quantum

improvement over previous approaches because: target

output voltage is directly resistor-programmable, regu-

lation is maintained well into discontinuous mode and

optional load compensation is available.

The Block Diagram shows an overall view of the system.

Many of the blocks are similar to those found in tradi-

tional designs including: internal bias regulator, oscilla-

tor, logic, current amplifier and comparator, driver and

output switch. The novel sections include a special

flyback error amplifier and a load compensation mecha-

nism. Also, due to the special dynamic requirements of

flyback control, the logic system contains additional

functionality not found in conventional designs.

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