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AN4003

PC POWER SUPPLY DESIGN WITH KA3511

Fairchild Semiconductor 

Ct

Feedback

Dead-time

control

Ck

Q

Q

Output Q1

Output Q2

Figure 4. Operating Waveform

6.3 DEADTIME CONTROL for SOFT-START

3mA

12

Vref

Remote

ON/OFF

DTC 19

+

C1

R1 22uF

47k

R2

1k

Figure 5. Soft-Start Circuit

Deadtime control for soft-start makes a power supply output rising time (Typ. 15ms) to reduce out-

put ringing voltage for 3.3V, 5V, and 12V. If output rising time is too fast, output ringing voltage

reaches OVP level.

You can make a soft start function by add external components R1, R2 and C1 (refer to figure 5).

At first the main power is turned-on, the deadtime control voltage keeps high state ( · = · 3V), and

then go to the low voltage( · = · 105mV) that devided by R1, R2.

VDTC LOW = R-----1---R--+--2---R-----2- × Vref(5V) = 104.9mV

Rev C, November 1999

10

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