12/9/04

Page 10 of 15

© 2003 Fairchild Semiconductor Corporation

OPTICALLY ISOLATED

ERROR AMPLIFIER

FOD2741A

FOD2741B

FOD2741C

The FOD2741

The FOD2741 is an optically isolated error amplifier. It incor-

porates 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 FOD2741 powers the secondary side, and

in particular provides the current to run the LED. The actual

structure of the FOD2741 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

+ 1.5V = 4.0V. This voltage can be generated either directly

from the output of the converter, or else from a slaved second-

ary 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

FOD2741 attempts to regulate its FB pin to the reference

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

Compensation

The compensation pin of the FOD2741 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 FOD2741. 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 10k

Ω resistor for the LED, the maximum current the

LED can see is (12V-4V) /10k

Ω = 800µA. The CTR of the

opto-isolator is a minimum of 100%, so the minimum collector

current of the photo-transistor when the diode is full on is also

800µA. The collector resistor must thus be such that:

select 12k

Ω to allow some margin.

R

TOP

R

BOTTOM

--------------------------

V

OUT

V

REF

--------------

1

–

=

V

OUT

2.5

–

R

TOP

-----------------------------

1040

µA

>

10V

5V

–

R

COLLECTOR

------------------------------------800

µA or R

COLLECTOR

6.25k

Ω;

>

<