AND8099/D

http://onsemi.com

4

Feedback Loop

The feedback loop is composed of an optocoupler, a shunt

regulator, a compensation capacitor and a resistor divider.

The optocoupler isolates the AC input from the DC output.

As a shunt regulator, the ON Semiconductor TL431 is used

to regulate the output voltage. This monolithic voltage

external resistors. It exhibits a wide current range of 1.0 mA

to 100 mA and is an excellent replacement for Zener diodes.

The reference voltage of the TL431 is set at 2.5 V, for a 5.0 V

output voltage, by a resistor divider R5 and R6 (low

tolerance, 2.0 k

W resistors). The TL431 monitors the 5.0 V

output voltage and compares the divided down voltage to its

2.5 V internal reference. A small increase in the output

voltage will cause the shunt regulator to start conducting,

thus sinking current through the optocoupler’s LED. In turn,

the optocoupler transistor becomes forward biased and starts

driving current into the control input pin of the NCP1055.

The power switch duty cycle is then adjusted accordingly. A

compensation capacitor C10 of 0.1

mF is placed between the

cathode and the reference pin of the TL431 for improved

stability. The resistor R3 limits the current going through the

optocoupler to a safe level and prevents damage to the

optocoupler.

+

+

+

+

+

C11

330

R6

2.0 k

R5

2.0 k

C10

0.1

R3

47

F1

2.0 A

C1

0.1

L1

10 mH

D1

D2

D3

D4

R1

91 k

+

C2

33

C3

220 p

C4

47 p

R2

2.2 k

D5

D6

T1

C7

330

C8

330

C9

330

C6

100 p

C5

10

+

IC2

R4

1.0 k

IC3

5.0 V, 2.0 A

C12

1.0

Figure 1. Circuit Diagram

NCP1055P100

X

C13

1n

L2

5.0

mH