The FOD2743 Optically Isolated Amplifier consists of the popular KA431 precision
programmable shunt reference and an optocoupler. The optocoupler is a gallium arsenide
(GaAs) light emitting diode optically coupled to a silicon phototransistor. It comes in 3
grades of reference voltage tolerance = 2%, 1%, and 0.5%.
The Current Transfer Ratio (CTR) ranges from 50% to 100%. It also has an outstanding
temperature coefficient of 50 ppm/°C. It is primarily intended for use as the error amplifier/
reference voltage/optocoupler function in isolated ac to dc power supplies and dc/dc converters.
When using the FOD2743, power supply designers can reduce the component count and
save space in tightly packaged designs. The tight tolerance reference eliminates the need
for adjustments in many applications. The device comes in a 8-pin dip white package.
OPTICALLY ISOLATED
ERROR AMPLIFIER
8
1
8
1
8
1
FEATURES
• Optocoupler, precision reference and error amplifier in single package
2. Functional operation under these conditions is not implied. Permanent damage may occur if the device is subjected to conditions
outside these ratings.
are defined as the differences between the maximum and minimum values
obtained over the rated temperature range. The average full-range temperature coefficient of the reference input voltage, ∆ V
is defined as:
A
OUT
25°C=(){}106×
| = ∆ V
COMP
I
. When the device is operating with two external resistors (see
LED
V
∆V
REF
where ∆ T
ppm/°C()
is the rated operating free-air temperature range of the device.
1. Device is considered as a two terminal device: Pins 1,2 3 and 4 are shorted together and Pins 5,6,7 and 8 are shorted together.
2. Common mode transient immunity at output high is the maximum tolerable (positive) dVcm/dt on the leading edge of the common mode impulse signal, Vcm, to assure that the output will remain high. Common mode transient immunity at output low is
the maximum tolerable (negative) dVcm/dt on the trailing edge of the common pulse signal,Vcm, to assure that the output will
remain low.
The FOD2743 is an optically isolated error amplifier. It incorporates three of the most common elements necessary to
make an isolated power supply, a reference voltage, an error
amplifier, and an optocoupler. It is functionally equivalent to
the popular KA431 shunt voltage 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 voltage 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 regulation,
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
FOD2743 attempts to regulate its FB pin to the reference
voltage, 2.5V. The ratio of the two resistors should thus be:
R
TOP
-------------------------R
BOTTOM
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
should be:
TOP
V
OUT
----------------------------R
TOP
V
OUT
--------------1–=
V
REF
2.5–
1040µA>
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
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 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 reference. If we
select a 10kΩ resistor for the LED, the maximum 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:
10V 5V–
-----------------------------------4 0 0 µA or R
R
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