FUJITSU MB3887 - Manual (Page 19)

MB3887
19
■ 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 (V
CC
≥
UVLO threshold voltage) , Q2
is turned off and the external soft-start capacitor (Cs) connected to the CS terminal is charged at 10
µ
A.
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
×
C
S
(
µ
F)
=
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 µ A 10 µ A
Q2
4.2 V
Error
Amp3
FB3
−
INE3
CS
C S
Soft-start circuit