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LT1425

Isolated Flyback Switching Regulator

Linear Technology 

LT1425

U

OPERATION

Within the dashed lines in the Block Diagram can be found

the RREF, RFB and ROCOMP resistors. They are external

resistors on the user-programmable LT1425. The capaci-

tor connected to the RCCOMP pin is also external.

The LT1425 operates much the same as traditional current

mode switchers, the major difference being a different

type of error amplifier which derives its feedback informa-

tion from the flyback pulse. Due to space constraints, this

discussion will not reiterate the basics of current mode

switcher/controllers and isolated flyback converters. A

good source of information on these topics is LTC’s

Application Note 19.

ERROR AMPLIFIER—PSEUDO DC THEORY

Please refer to the simplified diagram of the Flyback Error

Amplifier. Operation is as follows: when output switch Q4

turns off, its collector voltage rises above the VIN rail. The

amplitude of this flyback pulse, i.e., the difference between

it and VIN, is given as:

VFLBK

=

VOUT

+

VF + (ISEC)(ESR)

NSP

VF = D1 forward voltage

ISEC = Transformer secondary current

ESR = Total impedance of secondary circuit

NSP = Transformer effective secondary-to-primary

turns ratio

The flyback voltage is then converted to a current by the

action of RFB and Q1. Nearly all of this current flows

through resistor RREF to form a ground-referred voltage.

This is then compared to the internal bandgap reference by

the differential transistor pair Q2/Q3. The collector current

from Q2 is mirrored around and subtracted from fixed

current source IFXD at the VC pin. An external capacitor

integrates this net current to provide the control voltage to

set the current mode trip point.

The relatively high gain in the overall loop will then cause

the voltage at the RREF resistor to be nearly equal to the

bandgap reference VBG. (VBG is not present in final output

voltage setting equation. See Applications Information

section.) The relationship between VFLBK and VBG may

then be expressed as:

α

VFLBK

RFB

=

VBG

RREF

or,

) ) VFLBK = VBG

RFB

RREF

1

α

α = Ratio of Q1 IC to IE

VBG = Internal bandgap reference

Combination with the previous VFLBK expression yields an

expression for VOUT, in terms of the internal reference,

programming resistors, transformer turns ratio and diode

forward voltage drop:

) ) VOUT = VBG

RFB

RREF

NSP

α

– VF – ISEC (ESR)

Additionally, it includes the effect of nonzero secondary

output impedance. See Load Compensation for details.

The practical aspects of applying this equation for VOUT are

found in the Applications Information section.

So far, this has been a pseudo-DC treatment of flyback

error amplifier operation. But the flyback signal is a pulse,

not a DC level. Provision must be made to enable the

flyback amplifier only when the flyback pulse is present.

This is accomplished by the dashed line connections to the

block labeled “ENABLE.” Timing signals are then required

to enable and disable the flyback amplifier.

ERROR AMPLIFIER—DYNAMIC THEORY

There are several timing signals that are required for

proper LT1425 operation. Please refer to the Timing

Diagram.

Minimum Output Switch ON Time

The LT1425 effects output voltage regulation via flyback

pulse action. If the output switch is not turned on at all,

there will be no flyback pulse, and output voltage informa-

tion is no longer available. This would cause irregular loop

response and start-up/latchup problems. The solution

chosen is to require the output switch to be on for an

absolute minimum time per each oscillator cycle. This in

turn establishes a minimum load requirement to maintain

8

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