LIA136STR datasheet(9/13 Pages) IXYS

9 / 13 page

INTEGRATED CIRCUITS DIVISION

LIA135 / LIA13

R01

www.ixysic.com

2.1 Input Side Biasing

Power for the LIA135 / LIA136 error amplifier, voltage

reference, and optocoupler LED is applied to the LED

pin through a current limiting resistor. Typically, this

resistor’s voltage source is VOUT, the regulated power

supply output. For very low voltage designs where

VOUT lacks sufficient headroom to bias the input

circuitry, the resistor may be sourced from an auxiliary

secondary winding on the transformer. When using

the LIA135 / LIA136 this is an unlikely situation as

these devices were designed specifically to be used

for low voltage power supply applications. For all

implementations, the minimum bias voltage at the

LED pin is 1.6V.

There must be a current-limiting resistor (RLED) in

series with the LED pin to keep the current flow into

the device and through the LED within their respective

operating ranges for all expected supply output levels.

Although the value of RLED is determined in

conjunction with the value of the phototransistor’s

pull-up resistor RC, it’s minimum value is limited by the

maximum allowed input current. See Section 3.

Design Examples on page 10.

2.2 Supply Regulation

When connected as shown in the application circuit

above and properly configured, the LIA135 / LIA136

will regulate VOUT such that VFB is equal to VREF

(1.299V). To achieve this, the values of the voltage

divider resistors, R1 and R2, must be set in the

following manner:

Because VOUT regulation occurs when VFB =VREF any

change in bias current through R2 at the desired

regulated voltage level will cause a regulation error. As

shown in the Electrical Characteristics table the error

amplifier input at pin FB has an input bias current (IIB)

specification that reduces the current into R2. (IIB is

always into pin FB). This error causes the regulated

output voltage to increase which increases the current

through R1 by an amount equal to IIB, thereby

restoring the current through R2 to it’s original value.

Reducing the VOUT error created by the input bias

current to less than 1% is accomplished by setting the

value of R1 using the following formula:

Where 50

A is 100 x I

IB(max). This error can be

reduced to less than 0.05% by setting the current to

1mA. i.e. 2000 x IIB(max)

2.3 Compensation

Frequency response of the converter can be optimized

for the specific application by placing a compensation

network between the COMP and FB pins of the

LIA135 / LIA136. In a typical system with a

low-bandwidth requirement, only a 0.1µF capacitor

should be needed. For designs with more critical

bandwidth requirements, measurements of the loop

response must be made and compensation adjusted

as necessary.

2.4 Optocoupler Output Transistor

The output phototransistor of the LIA135 / LIA136

provides the isolated and amplified feedback signal

that represents the output of the converter. Typically,

the collector of the phototransistor will be pulled up by

a reference voltage provided by the power supply

control chip and the emitter will be grounded.

The base of the LIA135 output transistor is not

externally accessible. For the LIA136 however, the

base is brought out at pin 2 enabling the user to

extend the capabilities of the device beyond those of

the LIA135. Placing a resistor from the base to the

emitter extends the operational temperature range by

shunting base current around the base-emitter

junction thereby reducing dark current at elevated

temperatures. Immunity to large common mode

transients (CMTI) is enhanced by placing a capacitor

parallel to the base-emitter resistor. This shunts

transient currents around the base-emitter junction

rather than having them amplified by the transistor.

When using the LIA136 the base-emitter resistor must

be populated, otherwise the open base lead will pick

up atmospheric electromagnetic signals converting

them into noise components.

2.5 N/C Pins

The N/C (No Connect) pins have no internal

connection.

-------

OUT

REF

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





–

OUT

A

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



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