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 devices. 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 it’s 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.
V
out, bias
I
LED
R
L
• K
1
The output voltage
R
typically constrained by the user to
is
R
1
1
1
satisfy the design requirements of the application circuit.
R
2
K
3
Design considerations must also take into account that
R
L
affects the total gain and that CTR gains vary with
process. Nominally the
K
3
current should be around
LED
R
1
1-2mA but can be as high as 10-15mA if the user
requires.
LED current is limited by the resistor in series with
m
pin 8,
V
OUT
V
IN
•
the LED pin, to the supply and is typically 10-100 ohms
for operating currents of 1-2mA. The minimum operating
m
voltage of 2.74V for the LIA120 from pin 8 to pin 5 is
based on the sum of the voltage drop of the
R
CTR
FB
R
1
LED and
2
the operational voltage headroom of the 431. Minimum
R
1
R
2
operating voltage for the application circuit is therefore
the sum of the LIA120 minimum operating voltage plus
R
LED
C R
the voltage drop of the current limiting resistor
c
For
1
a
R
2
P
1
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
V
I
LED
two
out, bias
V
by setting
resistors. The application circuit in
K
IR
L
•
a
1 out, bias
divider feeding the FB pin, so
LED
Figure 6 shows resistor
R
L
• K
the operating conditions
1
for the gain are governed by:
R
1
R
1
1
R
1
R
1
1
R
2
K
3
R
2
K
3
K
3
is taken from the datasheet as 1 nominally. The ac
K
3
R
1
K
3
gain of the setup can
R
be represented by:
1
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
This calculation provides a more accurate gain
calculation but is only necessary when the voltage
V
out, bias
I
LED
divider resistor’s impedance is becoming close to the
R
L
• K
1
optical output impedance of the shunt regulator.
R
1
R
1
1
Compensation
R
2
K
3
The LIA120 is relatively easy to compensate but two
factors must be considered when analyzing the circuit.
K
3
The frequency response of the LIA120 can be as high
R
1
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
m
dominant pole
V
OUT
the linear
•
and
V
IN
optical coupler pole. The
open loop gain of the optical loop (for the application
diagram) is:
m
V
out, bias
CTR
FB
R
1
R
2
I
LED
R
1
R
2
R
L
K
1
The open
•
loop gain is affected by the selection of R
1
and R
2
and without any compensation the circuit may
R
LED
C
c
R
1
R
2
R
1
R
1
P
1
oscillate. The addition of
1
compensation network (C
c
a
R
2
K
3
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
K
3
R
1
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
m
V
OUT
V
IN
•
response is generally desired to be such that the circuit
will not oscillate for a large selection of R
1
and R
2
.
m
Therefore the compensation capacitor should not be less
than 100pF which gives adequate bandwidth for most
CTR
FB
R
1
R
2
designs. The bandwidth through the part will be:
R
1
R
2
R
LED
C
c
R
1
R
2
P
1
Where:
P
1
max is 1kHz (6.28krad/s) due to the internal
compensation of the 431.
CTR is the current transfer ratio of the feedback
optocoupler (0.001-0.003).
R
LED
is the combined impedance of the limiting resistor
and the LED resistance (25 ohms) and Gm is the
transconductance of the 431 (3 Siemens).
However, since some of these elements vary over
operating conditions and temperature, the bandwidth
should be practically limited to less than 40kHz to avoid
oscillations, which is the value computed by 100pF.
V
OUT
V
IN
•
V
OUT
V
IN
m
•
m
m
m
Rev. 2.0
CTR
FB
R
1
R
2
CTR
FB
R
1
R
2
R
1
R
2
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R R
7
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