LIA135 / LIA136

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R01

2. Functional Description

The LIA135 and LIA136 are optically isolated error

amplifiers. Each comprises the three primary

components necessary to implement feedback for an

isolated power supply in a single package. These

components are: an error amplifier; a voltage

reference; and an optocoupler. The LIA135 and

LIA136 are the functional equivalent of a 431 type

precision shunt regulator and an optocoupler in the

same package.

Understanding how the LIA135 and LIA136 function

and are used is best understood by referencing the

simplified application circuit shown in

Figure 10: Typical Isolated Power Supply Using an

the LIA135 / LIA136 is to sample the power supply

output to be regulated, generate an error signal, and

transmit the error signal across the isolation barrier to

the power supply’s control circuitry. Power for the input

side circuits, consisting of an LED; a shunt regulator;

and an error amplifier, is provided by the power

application circuit.

scaled sample of its voltage to pin FB, the error

amplifier’s non-inverting input. The error amplifier

high accuracy reference voltage and generates an

output which in turn sets the LED drive current.

internal reference voltage causes the error amplifier to

cause the optocoupler’s NPN output transistor to

conduct. Increasing the LED drive current results in an

which in turn decreases the voltage seen at the

The power supply’s control circuitry uses the error

voltage presented by the optocoupler’s output

transistor to interpret the power needs of the

secondary side load and to maintain regulation.

Figure 10: Typical Isolated Power Supply Using an Optical Feedback Error Amplifier

LIA135 / LIA136

+

_

+

_

LIA136

only

+

V

OUT

V

IN

PWM

Control

R1

R2

+

COMP

LED

FB

GND

C

E

B

V

CC

R

C

R

LED

I

F