AN4003 Datasheet, PDF (11/28 Pages) Fairchild Semiconductor – PC POWER SUPPLY DESIGN WITH KA3511

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 (1kΩ)) 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
+5V
+12V
R1
11kΩ
R2
33kΩ
R5
1kΩ
R3
2kΩ
C1
103
R4
1kΩ
COMP
2
E/A(+)
4
Vref
PWM Control
Comparator
3
E/A(-)
Err-Amp
1.25V
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.
Rev C, November 1999
11