FUJITSU MB39A104 - Manual (Page 14)

MB39A104
14
■ SETTING THE SOFT-START AND DISCHARGE TIMES
To prevent rush currents when the IC is turned on, you can set a soft-start by connecting soft-start capacitors
(C
S1
and C
S2
) to the CS1 terminal (pin 11) for channel 1 and the CS2 terminal (pin 14) for channel 2, respectively.
When CTL terminal (pin 24) goes to “H” level and IC starts (V
CC
≥ UVLO threshold voltage), the external soft-
start capacitors (C
S1
and C
S2
) connected to CS1 and CS2 terminals are charged at 10 µA. The error amplifier
output (FB1 (pin 9) , FB2 (pin 16) ) is determined by comparison between the lower one of the potentials at two
non-inverted input terminals (1.24 V, CS1 terminal voltages) and the inverted input terminal voltage (
−INE1 (pin
10) voltage, −INE2 (pin 15) voltage).
The FB1 (FB2) terminal voltage is decided for the soft-start period by the comparison between 1.24 V in an
internal reference voltage and the voltages of the CS1 (CS2) terminal. The DC/DC converter output voltage
rises in proportion to the CS1 (CS2) terminal voltage as the soft-start capacitor connected to the CS1 (CS2)
terminal is charged.
The soft-start time is obtained from the following formula:
Soft-start time: ts (time to output 100%)
ts (s) := 0.124
× C
S
(
µF)
t
:= 5 V
:= 1.24 V
:= 0 V
CS1 (CS2) terminal voltage
Error Amp block −INE1 (−INE2) voltage
Soft-start time (ts)