I
NTEGRATED
C
IRCUITS
D
IVISION
LIA135 / LIA136
10 www.ixysic.com R01
3. Design Examples
There are two basic designs, one with a discrete
resistor at the optocoupler output whose value can be
selected by the designer and the other with a resistor
integrated within the power supply’s PWM controller IC
whose value is fixed. Common to both design flows is
setting up the feedback from the power supply output
to the error amplifier input.
For the discrete optocoupler output resistor solution,
the design flow is determining the value of R
LED
and
then the optocoupler output resistor, R
C
. When the
optocoupler output resistor value is preselected and
fixed, the design procedure is reversed. In both cases
the value of these two resistors is dependent on the
other.
3.1 Error Amplifier Input Configuration
Regulation of the power supply’s output voltage is
accomplished by configuring the voltage divider
network consisting of R1 and R2 to apply a voltage
equal to the Reference Voltage (V
REF
) to FB, the
feedback amplifier’s input, when the supply’s output is
at it’s desired potential.
For this example, the nominal value of the power
supply output voltage is 1.8V. Because the LIA135 and
LIA136 operate with an input voltage of 1.6V there is
no requirement to provide an auxiliary transformer
winding to bias the optical feedback circuitry.
To reduce regulation error caused by loss of bias
current into pin FB, the current through R1 must be set
much greater than I
IB
, the leakage current into pin FB.
Setting R1 = 1k, a convenient common value will
ensure the current through R1 will be much greater
than I
IB
.
With R1 = 1k, and setting R2 = 2.61k will fix the
nominal supply output voltage to 1.797V, just slightly
below the target value of 1.8V.
3.2 Discrete Optocoupler Output Resistor
The value of the optocoupler’s collector pull-up resistor
(R
C
) and of the LED current-limiting resistor (R
LED
)
must be determined together with respect to the input
voltage range of the power supply’s PWM device.
Additionally, the operational range and performance
characteristics of the LIA135 and LIA136 must be
taken into account.
As an example, consider first that the minimum CTR of
the LIA135 / LIA136 is 500%. Selection of R
LED
to set
the minimum current through the LED (I
F
) to 1mA
when the converter output (V
OUT
) is at it’s nominal
value of 1.797V is as follows:
Using the nearest standard value that satisfies the
relationship above sets R
LED
= 178. Rearranging the
terms and calculating for the LED current gives
I
F
= 1.00674mA. A minimum of 5.0337mA 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 pull-up
resistor value is:
Setting R
C
= 1.40k (E96 standard value) changes
the collector voltage under these conditions from the
ideal 5V to 4.953V.
The value of R
LED
must never allow more than 20mA
of current to flow into the LED pin. Assuming a V
OUT
tolerance of 10% then:
The value R
LED
= 178 selected above satisfies the
minimum value of the LED resistor.
3.3 Integrated Optocoupler Output Resistor
Many times the collector pull-up resistor is integrated
into the PWM controller IC and may be a current
source rather than a resistive component.
The design methodology is similar to the external
discrete pull-up resistor design but the LED current
limiting resistor must be calculated starting from the
pull-up at the optocoupler output transistor’s collector.
R
LED
V
OUT
V
LED min
–
I
F
I
Qmax
+
-------------------------------------------- -
R
LED
1.797V 1.6V–
1mA 100
A+
---------------------------------- -
0.197V
1.1mA
----------------- 179.09==
R
C
12V 5V–
5.0337mA
------------------------ -
1.391k=
R
LED
1.98V 1.6V–
20mA
------------------------------- -
19=
