©2004 Fairchild Semiconductor Corporation www.fairchildsemi.com
FOD2743A, FOD2743B, FOD2743C Rev. 1.0.1 11
FOD2743A, FOD2743B, FOD2743C — Optically Isolated Error Amplifier
The FOD2743
The FOD2743 is an optically isolated error amplifier. It
incorporates three of the most common elements neces-
sary to make an isolated power supply, a reference volt-
age, an error amplifier, and an optocoupler. It is
functionally equivalent to the popular KA431 shunt volt-
age regulator plus the CNY17F-X optocoupler.
Powering the Secondary Side
The LED pin in the FOD2743 powers the secondary
side, and in particular provides the current to run the
LED. The actual structure of the FOD2743 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 volt-
age applied to the LED pin is thus 2.5V + 1.2V = 3.7V.
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 regula-
tion, 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 cur-
rent 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 FOD2743 attempts to regulate its FB pin to the ref-
erence voltage, 2.5V. The ratio of the two resistors
should thus be:
The absolute value of the top resistor is set by the input
offset current of 5.2µA. To achieve 0.5% accuracy, the
resistance of R
TOP
should be:
Compensation
The compensation pin of the FOD2743 provides the
opportunity 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 refer-
ence 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 FOD2743. In a
normal configuration the collector will be attached to a
pull-up resistor and the emitter grounded. There is no
base connection necessary.
The value of the pull-up resistor, and the current limiting
resistor 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
voltage feeding the collector pull-up is +10V, and the
PWM IC is the Fairchild FAN4803, which has a 5V refer-
ence. If we select a 10kΩ resistor for the LED, the maxi-
mum current the LED can see is:
(12V – 4V) / 10kΩ = 800µA.
The CTR of the opto-isolator is a minimum of 50%, so
the minimum collector current of the photo-transistor
when the diode is full on is 400µA. The collector resistor
must thus be such that:
select 20kΩ to allow some margin.
R
TOP
R
BOTTOM
--------------------------
V
OUT
V
REF
-------------- 1–=
V
OUT
2.5–
R
TOP
-----------------------------
1040µA>
10V 5V–
R
COLLECTOR
----------------------------------- 4 0 0 µA or R
COLLECTOR
12.5kΩ;><
