MB39A104
■ 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
(CS1 and CS2) 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 (VCC ≥ UVLO threshold voltage), the external soft-
start capacitors (CS1 and CS2) 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 × CS (µF)
CS1 (CS2) terminal voltage
=: 5 V
Error Amp block −INE1 (−INE2) voltage
=: 1.24 V
=: 0 V
t
Soft-start time (ts)
14
FUJITSU [ FUJITSU ]
