
LIA120
www.clare.com 7
Rev. 1.0
The LIA120
The LIA120 is an optically isolated error amplifier. It
incorporates three of the most common elements
necessary to make an isolated power supply: a
reference voltage, and error amplifier, and a linear
optocoupler. It is functionally equivalent to the IX431
shunt regulator plus a linear optocoupler.
Powering the isolated input
The isolated input of the LIA120 is powered through
the LED pin (pin 8) through the part to it’s isolated
ground. The typical operating current in the device
is dictated by the output voltage and output current
requirements, and the CTR of the linear optocoupler.
The output voltage is typically constrained by the
user, RL also affects the total gain, and CTR varies
with process, but the nominal LED current should be
around 1-2mA, and can be as high as 10mA-15mA if
the user requires.
The LED current is limited by a resistor in series with
the LED pin to the supply and is typically 10-100
ohms for operating currents of 1-2mA. The current
limiting resistor, in conjunction with the LED voltage,
and voltage headroom of the IX431 sets the minimum
operating voltage of the isolated input to 2.74V plus
the drop of the limiting resistor.
Feedback
Setting the gain for the LIA120 is accomplished simply
by setting two resistors. The application circuit shows
a resistor divider feeding the FB pin, so the operating
conditions for the gain are governed by:
K3 is taken from the datasheet as 1 nominally. The
ac gain of the setup can be represented by:
Where:
• Gm = 1/ZKA which is ~ 3 Siemens
• CTRFB is approximately CTRForward = 0.02 nominally
CTRFB/CTRForward = K3
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 signals.
In the localized optical feedback there are two poles
to consider, the IX431 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 R1
and R2 and without any compensation the circuit may
oscillate. The addition of a compensation network
(Cc and Rc) control the maximum bandwidth so that
open 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 limited to typically
<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 R1 and
R2. Therefore the compensation capacitor should not
be less than 100pF which gives adequate bandwidth
for most designs. The bandwidth through the part will
be:
Where:
P1 max is 1KHz (= 6.28Krad/s), due to the internal
compensation of the IX431
CTR is the current transfer ratio of the feedback
optocoupler (0.025 – 0.005)
RLED is the combined impedance of the limiting
resistor and the LED resistance (25 Ohms)
Gm is transconductance of the IX431 (3 Siemens)
However, since some of these elements vary over
operating conditions and temperature, the bandwidth
should be practically limited to <40KHz to avoid
oscillations, which is the value computed by 100pF.
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.