Power Supply User's Manual

MB3887
19
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METHOD OF SETTING THE SOFT-START TIME
For preventing rush current upon activation of IC, the IC allows soft-start using the capacitor (Cs) connected to
the CS terminal (pin 22) .
When CTL terminal (pin 14) is placed under “H” level and IC is activated (VCC ≥ UVLO threshold voltage) , Q2
is turned off and the external soft-start capacitor (Cs) connected to the CS terminal is charged at 10 µA.
Error Amp output (FB3 terminal (pin 15) ) is determined by comparison between the lower voltage of the two
non-reverse input terminals (4.2 V and CS terminal voltage) and reverse input terminal voltage (−INE3 terminal
(pin 16) voltage) . Within the soft-start period (CS terminal voltage < 4.2 V) , FB3 is determined by comparison
between −INE3 terminal voltage and CS terminal voltage, and DC/DC converter output voltage goes up propor-
tionately with the increase of CS terminal voltage caused by charging on the soft-start capacitor.Soft-start time
is found by the following formula :
Soft-start time : ts (time to output 100 %)
t
S (s) := 0.42 × CS (µF)
= 4.9 V
= 4.2 V
= 0 V
CS terminal voltage
Comparison with Error Amp block − INE3
voltage.
Soft-start time: ts
15
16
22
−
+
+
UVLO
VREF
10 µA 10 µA
Q2
4.2 V
Error
Amp3
FB3
−INE3
CS
C
S
Soft-start circuit