9/6/02
Page 10 of 14
© 2002 Fairchild Semiconductor Corporation
OPTICALLY ISOLATED
ERROR AMPLIFIER
FOD2711
The FOD2711
The FOD2711 is an optically isolated error amplifier. It incorpo-
rates three of the most common elements necessary to make
an isolated power supply, a reference voltage, an error ampli-
fier, and an optocoupler. It is functionally equivalent to the
popular RC431A shunt voltage regulator plus the CNY17F-3
optocoupler.
Powering the Secondary Side
The LED pin in the FOD2711 powers the secondary side, and
in particular provides the current to run the LED. The actual
structure of the FOD2711 dictates the minimum voltage that
can be applied to the LED pin: The error amplifier output has a
minimum of the reference voltage, and the LED is in series
with that. Minimum voltage applied to the LED pin is thus
1.24V + 1.5V = 2.74V. This voltage can be generated either
directly from the output of the converter, or else from a slaved
secondary winding. The secondary winding will not affect regu-
lation, as the input to the FB pin may still be taken from the
output winding.
The LED pin needs to be fed through a current limiting resistor.
The value of the resistor sets the amount of current through
the LED, and thus must be carefully selected in conjunction
with the selection of the primary side resistor.
Feedback
Output voltage of a converter is determined by selecting a
resistor divider from the regulated output to the FB pin. The
FOD2711 attempts to regulate its FB pin to the reference
voltage, 1.24V. The ratio of the two resistors should thus be:
The absolute value of the top resistor is set by the input offset
current of 0.8µA. To achieve 1% accuracy, the resistance of
R
TOP
should be:
Compensation
The compensation pin of the FOD2711 provides the opportu-
nity for the designer to design the frequency response of the
converter. A compensation network may be placed between
the COMP pin and the FB pin. In typical low-bandwidth
systems, a 0.1µF capacitor may be used. For converters with
more stringent requirements, a network should be designed
based on measurements of the system’s loop. An excellent
reference for this process may be found in “Practical Design of
Power Supplies” by Ron Lenk, IEEE Press, 1998.
Secondary Ground
The GND pin should be connected to the secondary ground of
the converter.
No Connect Pins
The NC pins have no internal connection. They should not
have any connection to the secondary side, as this may
compromise the isolation structure.
Photo-Transistor
The Photo-transistor is the output of the FOD2711. In a normal
configuration the collector will be attached to a pull-up resistor
and the emitter grounded. There is no base connection neces-
sary.
The value of the pull-up resistor, and the current limiting resis-
tor feeding the LED, must be carefully selected to account for
voltage range accepted by the PWM IC, and for the variation in
current transfer ratio (CTR) of the opto-isolator itself.
Example: The voltage feeding the LED pins is +12V, the volt-
age feeding the collector pull-up is +10V, and the PWM IC is
the Fairchild KA1H0680, which has a 5V reference. If we
select a 10KΩ resistor for the LED, the maximum current the
LED can see is (12V-2.74V) /10KΩ = 926µA. The CTR of the
opto-isolator is a minimum of 100%, and so the minimum
collector current of the photo-transistor when the diode is full
on is also 926µA. The collector resistor must thus be such that:
select 10KΩ to allow some margin.
R
TOP
R
BOTTOM
--------------------------
V
OUT
V
REF
-------------- 1–=
V
OUT
1.24–
R
TOP
-------------------------------- 8 0 µA>
10V 5V–
R
COLLECTOR
------------------------------------926µA or R
COLLECTOR
5.4KΩ;><
