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7
R04
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
L
affects the total gain and that CTR
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
LED
=
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
2
, and without any compensation the circuit may
oscillate. The addition of a compensation network (C
c
and R
c
) 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 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
1
and
R
2
. Therefore the compensation capacitor should not
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
3
is taken from the datasheet as 1 nominally. The AC
gain of the setup can be represented by:
Where:
• G
m
= 1/Z
OUT
which is ~ 3 Siemens
• CTR
FB
is approximately CTR
Forward
= 0.02 nominally
CTR
FB
= K
1
, CTR
FORWARD
= K
2
, CTR
FORWARD
/CTR
FB
= K
3
A
V
V
OUT
/V
IN
=
R
L
R
1
R
2
R
1
G
m
CTR
FB
1
G
m
CTR
Forward
1
Av, OPTICAL
=
Gm CTR
FB
R
1
R
2
