
PRELIMINARY
I
NTEGRATED
C
IRCUITS
D
IVISION
PRELIMINARY
8R00D
LIA130
The LIA130
The LIA130 is essentially an optically isolated
error amplifier. It comprises three of the necessary
components to form an isolated power supply: an
optocoupler, an error amplifier, and a reference
voltage device. The LIA130 is the functional equivalent
of a 431 series shunt voltage regulator plus an
optocoupler in the same package.
LED Pin
FB Pin
COMP Pin
GND Pin
Connect the GND pin of the LIA130 to the secondary
ground of the converter.
NC Pins
The NC (not connected) pins have no internal
connection and must not have any connection to
the secondary side, as this might compromise the
primary-to-secondary isolation.
C & E Pins
The LED within the LIA130 is powered by a sample of
the output voltage that is being regulated. Typically, a
resistor divider is provided to keep this voltage sample
within the operating range of the LED and its series
resistor. As the output voltage changes, the LED light
output changes, which provides a changing error
voltage from the phototransistor output of the LIA130.
The sampled voltage must be at least 1.24V (the
reference voltage) plus 1.5V (the LED voltage drop)
or a minimum of 2.74 volts. The sampled voltage can
also be provided from a slaved secondary winding of
the transformer rather than a resistor divider.
There must be a current-limiting resistor in series with
the LED pin to keep the current flow through the LED
within its operating range for all expected sampled
output levels. This resistor must be selected along with
the resistor in series with the output phototransistor.
The LIA130, when connected as shown in the Typical
Application Circuit, will regulate the output voltage so
that the voltage on its FB pin is 1.24V. Set the values
of the two voltage divider resistors, R1 and R2 in this
way:
R1/R2 = (VOUT / VREF) - 1
The value of R1 is set by the input offset current,
0.8A. 1% accuracy is obtained when the value of R1
satisfies this formula:
((VOUT - 1.24) / R1) > 80µA
The frequency response of the converter can be
optimized for the particular application by placing a
compensation network between the COMP pin and
the FB pin of the LIA130. In a system with a typical
low-bandwidth requirement, only a 0.1µF capacitor
might be needed.
If the system has more critical bandwidth
requirements, then measurements must be made of
the system's loop. See “Practical Design of Power
Supplies” by Ron Lenk, IEEE Press, 1998, for an
excellent description.
The output phototransistor of the LIA130 provides the
isolated and amplified error signal that represents the
DC output level of the converter. Typically, the collector
of the phototransistor will be pulled up to voltage and
the emitter will be grounded.
The value of the collector's pull-up resistor and the
value of the LED current-limiting resistor must be
determined together with respect to the input voltage
range of the PWM circuitry. The variation in CTR of
the LIA130 must also be taken into account.
As an example, consider first that the minimum CTR
of the LIA130 is 300%. If the current-limiting resistor
of the LED is set to allow a maximum current through
the LED of 1mA when the converter output is at a
nominal 15 volts:
R
LED = (15V - 2.74V) / 0.001A = 12.260k
then a minimum of 3mA 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 resistor value is:
R
PULLUP > (12V - 5V) / 0.003A > 2.333k
[Standard values can be selected for RLED and
RPULLUP and the small differences then re-calculated.]