11

Rev C, November 1999

So Output Duty Ratio will change from the minimum duty ratio to the maximum duty ratio.

Also, if the remote voltage is high, the deadtime control voltage will keep 3V (=3mA xR2 (1k

Ω)) by

the internal 3mA current source for soft start. Therefore, when the remote voltage is low, the dead-

time control voltage will be changed from 3V to almost ground potential. And its soft start time

dependent on external capacitor C1.

6.4 OUTPUT VOLTAGE REGULATION

Figure 6. Output Regulation Circuit

+5V/+12V output voltages are determined by resistor ratio of R1,R2,R3 and R4. The resistor value

can be changed by set condition and requirements.

R5, C1 are the compensation circuit for stability.

If output voltage (+5V or +12V) is increase, duty ratio of main power switch will be reduced by

PWM control comparator signal and error amplifier output. Therefore the output voltage will be

reduced.

On the contrary, if output voltage (+5V or +12V) is reduce, duty ratio of main power switch will be

increased by PWM control comparator signal and error amplifier output. Therefore the output volt-

age will be increased. So the output voltage of power supply will be regulated.

2

4

3

+5V

+12V

COMP

E/A(-)

E/A(+)

R5

1k

Ω

R2

33kΩ

R1

11kΩ

R3

2k

Ω

R4

1k

Ω

C1

103

Vref

PWM Control

Comparator

1.25V

Err-Amp