LT1737IGN datasheet(9/28 Pages) LINER

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LT1737

1737f

The LT1737 is a current mode switcher controller IC

designed specifically for the isolated flyback topology. The

Block Diagram shows an overall view of the system. Many

of the blocks are similar to those found in traditional

designs, including: Internal Bias Regulator, Oscillator,

Logic, Current Amplifier and Comparator, Driver and Out-

put Switch. The novel sections include a special Flyback

Error Amplifier and a Load Compensation mechanism.

Also, due to the special dynamic requirements of flyback

control, the Logic system contains additional functionality

not found in conventional designs.

The LT1737 operates much the same as traditional current

mode switchers, the major difference being a different

type of error amplifier that 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 Application

Note AN19.

ERROR AMPLIFIER—PSEUDO DC THEORY

Please refer to the simplified diagram of the Flyback Error

Amplifier. Operation is as follows: when MOSFET output

switch M1 turns off, its drain voltage rises above the VIN

rail. The amplitude of this flyback pulse as seen on the third

winding is given as:

ESR

FLBK

OUT

SEC

()

•

VF = D1 forward voltage

ISEC = transformer secondary current

ESR = total impedance of secondary circuit

NST = transformer effective secondary-to-third

winding turns ratio

The flyback voltage is then scaled by external resistor

divider R1/R2 and presented at the FB pin. This is then

compared to the internal bandgap reference by the differ-

ential transistor pair Q1/Q2. The collector current from Q1

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.

OPERATIO

The relatively high gain in the overall loop will then cause

the voltage at the FB pin to be nearly equal to the bandgap

reference VBG. The relationship between VFLBK and VBG

may then be expressed as:

FLBK

()

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:

ESR

OUT

SEC

()







––

•

Additionally, it includes the effect of nonzero secondary

output impedance, which is discussed in further detail, see

Load Compensation Theory. The practical aspects of

applying this equation for VOUT are found in the Applica-

tions 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 dotted line connections to the

block labeled “ENAB”. 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 LT1737 operation. Please refer to the Timing

Diagram.

Minimum Output Switch On Time

The LT1737 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 cho-

sen is to require the output switch to be on for an absolute

minimum time per each oscillator cycle. This in turn estab-

lishes a minimum load requirement to maintain regula-

tion. See Applications Information for further details.

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