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7

R03

LIA120

The LIA120

The LIA120 is an optically-coupled isolated linear error

amplifier. It integrates three of the most fundamental

elements necessary to make an isolated power supply:

a reference voltage, an error amplifier, and an isolated

coupling device. It is functionally equivalent to a 431

type shunt regulator plus a linear optical amplifier.

Powering the Isolated Input

The isolated input of the LIA120 is powered through

the LED pin (pin 8) via the part to its isolated

ground at pin 5. The typical operating current of

the device is determined by the output voltage and

current requirements as well as the CTR of the

linear optocoupler. For Figure 7, the LED current

requirement is set by the following equation.

The output voltage is typically constrained by the user

to satisfy the design requirements of the application

circuit. Design considerations must also take into

account that R

gains vary with process. Nominally the LED current

should be around 1-2mA but can be as high as

10-15mA if the user requires.

LED current is limited by the resistor in series with pin

8, the LED pin, to the supply and is typically 10-100

ohms for operating currents of 1-2mA. The minimum

operating voltage of 2.74V for the LIA120 from pin 8

to pin 5 is based on the sum of the voltage drop of the

LED and the operational voltage headroom of the 431.

Minimum operating voltage for the application circuit

is therefore the sum of the LIA120 minimum operating

voltage plus the voltage drop of the current limiting

resistor For a design with 1mA of LED current and

a current limiting resistor of 100 ohms, the minimum

operating voltage is calculated to be 2.74 + (0.001)

(100) = 2.84V.

Feedback

Setting the gain for the LIA120 is accomplished simply

by setting two resistors. The application circuit in

Figure 6 shows a resistor divider feeding the FB pin,

so the operating conditions for the gain are governed

by:

I

V

out,bias

R

L

K

1

R

1

R

2

Vin

Vref

Vref

Vout

R

1

R

L

1

K

3

-1

=

-

Compensation

The LIA120 is relatively easy to compensate but two

factors must be considered when analyzing the circuit.

The frequency response of the LIA120 can be as

high as 40kHz, but must be limited because of the

closed loop optical feedback to the input signal. In

the localized optical feedback there are two poles to

consider, the 431 dominant pole and the linear optical

coupler pole. The open loop gain of the optical loop

(for the application diagram) is:

The open loop gain is affected by the selection of R

1

and R

oscillate. The addition of a compensation network (C

c

and R

loop gain is rolling off long before the optical pole

causes the circuit to oscillate. The optical pole is at

~180kHz so the bandwidth is typically limited to less

than 40kHz.

While there is flexibility in the part to change the

compensation technique, the upper limit on frequency

response is generally desired to be such that the

circuit will not oscillate for a large selection of R

R

be less than 100pF, which gives adequate bandwidth

for most designs.

This calculation provides a more accurate gain

calculation, but is only necessary when the voltage

divider resistor’s impedance is becoming close to the

optical output impedance of the shunt regulator.

K

gain of the setup can be represented by:

Where:

• G

• CTR

CTR

A

V

V

=

R

L

R

1

R

2

R

1

G

1

G

1

FB

R

1

R

2