Ct
Feedback
Dead-time
control
Ck
Q
Q
Output Q1
Output Q2
Figure 4. Operating Waveform
6.3 DEADTIME CONTROL for SOFT-START
12
Vref
3mA
+
C1
22uF
R1
47k
19
DTC
R2
1k
Remote
ON/OFF
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.
R2
= --------------------- ×
VDTC LOW
Vref(5V) = 104.9mV
+
R1 R2
Rev C, November 1999
10
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
