LIA135 / LIA136

10

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R01

3. Design Examples

There are two basic designs, one with a discrete

resistor at the optocoupler output whose value can be

selected by the designer and the other with a resistor

integrated within the power supply’s PWM controller IC

whose value is fixed. Common to both design flows is

setting up the feedback from the power supply output

to the error amplifier input.

For the discrete optocoupler output resistor solution,

optocoupler output resistor value is preselected and

fixed, the design procedure is reversed. In both cases

the value of these two resistors is dependent on the

other.

3.1 Error Amplifier Input Configuration

Regulation of the power supply’s output voltage is

accomplished by configuring the voltage divider

network consisting of R1 and R2 to apply a voltage

feedback amplifier’s input, when the supply’s output is

at it’s desired potential.

For this example, the nominal value of the power

supply output voltage is 1.8V. Because the LIA135 and

LIA136 operate with an input voltage of 1.6V there is

no requirement to provide an auxiliary transformer

winding to bias the optical feedback circuitry.

To reduce regulation error caused by loss of bias

current into pin FB, the current through R1 must be set

Setting R1 = 1k

, a convenient common value will

ensure the current through R1 will be much greater

With R1 = 1k

, and setting R2 = 2.61k will fix the

nominal supply output voltage to 1.797V, just slightly

below the target value of 1.8V.

3.2 Discrete Optocoupler Output Resistor

The value of the optocoupler’s collector pull-up resistor

must be determined together with respect to the input

voltage range of the power supply’s PWM device.

Additionally, the operational range and performance

characteristics of the LIA135 and LIA136 must be

taken into account.

As an example, consider first that the minimum CTR of

value of 1.797V is as follows:

Using the nearest standard value that satisfies the

terms and calculating for the LED current gives

through the collector pull-up resistor. If the collector is

pulled up to 12V and the PWM has an internal

reference voltage of 5V, then the minimum pull-up

resistor value is:

the collector voltage under these conditions from the

ideal 5V to 4.953V.

tolerance of 10% then:

minimum value of the LED resistor.

3.3 Integrated Optocoupler Output Resistor

Many times the collector pull-up resistor is integrated

into the PWM controller IC and may be a current

source rather than a resistive component.

The design methodology is similar to the external

discrete pull-up resistor design but the LED current

limiting resistor must be calculated starting from the

pull-up at the optocoupler output transistor’s collector.

R

LED

V

OUT

V

LED min



–

I

F

I

Qmax



+

---------------------------------------------



R

LED

1.797V

1.6V

–

1mA

100

+

-----------------------------------



0.197V

1.1mA

-----------------

179.09



==

R

C

12V

5V

–

5.0337mA

-------------------------



1.391k



=

R

LED

1.98V

1.6V

–

20mA

--------------------------------



19



=