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LT1737IS

High Power Isolated Flyback Controller

Linear Technology 

LT1737

U

OPERATIO

Enable Delay

When the output switch shuts off, the flyback pulse

appears. However, it takes a finite time until the trans-

former primary side voltage waveform approximately rep-

resents the output voltage. This is partly due to finite rise

time on the MOSFET drain node, but more importantly,

due to transformer leakage inductance. The latter causes

a voltage spike on the primary side not directly related to

output voltage. (Some time is also required for internal

settling of the feedback amplifier circuitry.)

In order to maintain immunity to these phenomena, a fixed

delay is introduced between the switch turnoff command

and the enabling of the feedback amplifier. This is termed

enable delay. In certain cases where the leakage spike is

not sufficiently settled by the end of the enable delay

period, regulation error may result. See Application Infor-

mation for further details.

Collapse Detect

Once the feedback amplifier is enabled, some mechanism

is then required to disable it. This is accomplished by a

collapse detect comparator, which compares the flyback

voltage (FB referred) to a fixed reference, nominally 80%

of VBG. When the flyback waveform drops below this

level, the feedback amplifier is disabled. This action

accommodates both continuous and discontinuous mode

operation.

Minimum Enable Time

The feedback amplifier, once enabled, stays enabled for a

fixed minimum time period termed “minimum enable

time.” This prevents lockup, especially when the output

voltage is abnormally low, e.g., during start-up. The mini-

mum enable time period ensures that the VC node is able

to “pump up” and increase the current mode trip point to

the level where the collapse detect system exhibits proper

operation. The “minimum enable time” often determines

the low load level at which output voltage regulation is lost.

See Applications Information for details.

Effects of Variable Enable Period

It should now be clear that the flyback amplifier is enabled

during only a portion of the cycle time. This can vary from

the fixed “minimum enable time” described to a maximum

of roughly the “off” switch time minus the enable delay

time. Certain parameters of flyback amp behavior will then

be directly affected by the variable enable period. These

include effective transconductance and VC node slew rate.

LOAD COMPENSATION THEORY

The LT1737 uses the flyback pulse to obtain information

about the isolated output voltage. A potential error source

is caused by transformer secondary current flow through

the real life nonzero impedances of the output rectifier,

T1

R1

FB

R2 LOAD

COMP I

IM

Q1 Q2

VBG Q3

A1

IM

ROCMP

VIN

M1

R3

50k

ISENSE

RCMPC

RSENSE

10

1737 F01

Figure 1. Load Compensation Diagram

1737f

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