I
NTEGRATED
C
IRCUITS
D
IVISION
LIA135 / LIA13
R01 www.ixysic.com 9
2.1 Input Side Biasing
Power for the LIA135 / LIA136 error amplifier, voltage
reference, and optocoupler LED is applied to the LED
pin through a current limiting resistor. Typically, this
resistor’s voltage source is V
OUT,
the regulated power
supply output. For very low voltage designs where
V
OUT
lacks sufficient headroom to bias the input
circuitry, the resistor may be sourced from an auxiliary
secondary winding on the transformer. When using
the LIA135 / LIA136 this is an unlikely situation as
these devices were designed specifically to be used
for low voltage power supply applications. For all
implementations, the minimum bias voltage at the
LED pin is 1.6V.
There must be a current-limiting resistor (R
LED
) in
series with the LED pin to keep the current flow into
the device and through the LED within their respective
operating ranges for all expected supply output levels.
Although the value of R
LED
is determined in
conjunction with the value of the phototransistor’s
pull-up resistor R
C
, it’s minimum value is limited by the
maximum allowed input current. See Section 3.
Design Examples on page 10.
2.2 Supply Regulation
When connected as shown in the application circuit
above and properly configured, the LIA135 / LIA136
will regulate V
OUT
such that V
FB
is equal to V
REF
(1.299V). To achieve this, the values of the voltage
divider resistors, R1 and R2, must be set in the
following manner:
Because V
OUT
regulation occurs when V
FB
=V
REF
any
change in bias current through R2 at the desired
regulated voltage level will cause a regulation error. As
shown in the Electrical Characteristics table the error
amplifier input at pin FB has an input bias current (I
IB
)
specification that reduces the current into R2. (I
IB
is
always into pin FB). This error causes the regulated
output voltage to increase which increases the current
through R1 by an amount equal to I
IB
, thereby
restoring the current through R2 to it’s original value.
Reducing the V
OUT
error created by the input bias
current to less than 1% is accomplished by setting the
value of R1 using the following formula:
Where 50A is 100 x I
IB(max)
. This error can be
reduced to less than 0.05% by setting the current to
1mA. i.e. 2000 x I
IB(max)
2.3 Compensation
Frequency response of the converter can be optimized
for the specific application by placing a compensation
network between the COMP and FB pins of the
LIA135 / LIA136. In a typical system with a
low-bandwidth requirement, only a 0.1µF capacitor
should be needed. For designs with more critical
bandwidth requirements, measurements of the loop
response must be made and compensation adjusted
as necessary.
2.4 Optocoupler Output Transistor
The output phototransistor of the LIA135 / LIA136
provides the isolated and amplified feedback signal
that represents the output of the converter. Typically,
the collector of the phototransistor will be pulled up by
a reference voltage provided by the power supply
control chip and the emitter will be grounded.
The base of the LIA135 output transistor is not
externally accessible. For the LIA136 however, the
base is brought out at pin 2 enabling the user to
extend the capabilities of the device beyond those of
the LIA135. Placing a resistor from the base to the
emitter extends the operational temperature range by
shunting base current around the base-emitter
junction thereby reducing dark current at elevated
temperatures. Immunity to large common mode
transients (CMTI) is enhanced by placing a capacitor
parallel to the base-emitter resistor. This shunts
transient currents around the base-emitter junction
rather than having them amplified by the transistor.
When using the LIA136 the base-emitter resistor must
be populated, otherwise the open base lead will pick
up atmospheric electromagnetic signals converting
them into noise components.
2.5 N/C Pins
The N/C (No Connect) pins have no internal
connection.
R1
R2
------ -
V
OUT
V
REF
-------------
1–=
R1
V
OUT
50A
-------------
