PRELIMINARY

PRELIMINARY

8

R00D

LIA130

The LIA130

The LIA130 is essentially an optically isolated

error amplifier. It comprises three of the necessary

components to form an isolated power supply: an

optocoupler, an error amplifier, and a reference

voltage device. The LIA130 is the functional equivalent

of a 431 series shunt voltage regulator plus an

optocoupler in the same package.

LED Pin

FB Pin

COMP Pin

GND Pin

Connect the GND pin of the LIA130 to the secondary

ground of the converter.

NC Pins

The NC (not connected) pins have no internal

connection and must not have any connection to

the secondary side, as this might compromise the

primary-to-secondary isolation.

C & E Pins

The LED within the LIA130 is powered by a sample of

the output voltage that is being regulated. Typically, a

resistor divider is provided to keep this voltage sample

within the operating range of the LED and its series

resistor. As the output voltage changes, the LED light

output changes, which provides a changing error

voltage from the phototransistor output of the LIA130.

The sampled voltage must be at least 1.24V (the

reference voltage) plus 1.5V (the LED voltage drop)

or a minimum of 2.74 volts. The sampled voltage can

also be provided from a slaved secondary winding of

the transformer rather than a resistor divider.

There must be a current-limiting resistor in series with

the LED pin to keep the current flow through the LED

within its operating range for all expected sampled

output levels. This resistor must be selected along with

the resistor in series with the output phototransistor.

The LIA130, when connected as shown in the Typical

Application Circuit, will regulate the output voltage so

that the voltage on its FB pin is 1.24V. Set the values

of the two voltage divider resistors, R1 and R2 in this

way:

R1/R2 = (V

The value of R1 is set by the input offset current,

0.8

A. 1% accuracy is obtained when the value of R1

satisfies this formula:

((V

The frequency response of the converter can be

optimized for the particular application by placing a

compensation network between the COMP pin and

the FB pin of the LIA130. In a system with a typical

low-bandwidth requirement, only a 0.1µF capacitor

might be needed.

If the system has more critical bandwidth

requirements, then measurements must be made of

the system's loop. See “Practical Design of Power

Supplies” by Ron Lenk, IEEE Press, 1998, for an

excellent description.

The output phototransistor of the LIA130 provides the

isolated and amplified error signal that represents the

DC output level of the converter. Typically, the collector

of the phototransistor will be pulled up to voltage and

the emitter will be grounded.

The value of the collector's pull-up resistor and the

value of the LED current-limiting resistor must be

determined together with respect to the input voltage

range of the PWM circuitry. The variation in CTR of

the LIA130 must also be taken into account.

As an example, consider first that the minimum CTR

of the LIA130 is 300%. If the current-limiting resistor

of the LED is set to allow a maximum current through

the LED of 1mA when the converter output is at a

nominal 15 volts:

R

then a minimum of 3mA will flow 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 resistor value is:

R

[Standard values can be selected for R

R