MB39A104
14
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SETTING THE SOFT-START AND DISCHARGE TIMES
To prevent rush currents when the IC is tur ned on, you can set a soft-star t by connecting soft-star t capacitors
(C
S1 and CS2) to the CS1 terminal (pin 11) for channel 1 and the CS2 terminal (pin 14) for channel 2, respectively.
When CTL ter minal (pin 24) goes to “H” level and IC star ts (V
CC ≥ UVLO threshold voltage), the exter nal soft-
star t capacitors (C
S1 and CS2) connected to CS1 and CS2 ter minals are charged at 10 µA. The error amplifier
output (FB1 (pin 9) , FB2 (pin 16) ) is determined by comparison between the lo wer one of the potentials at tw o
non-inv erted input terminals (1.24 V, CS1 terminal voltages) and the in v erted input terminal voltage (−INE1 (pin
10) voltage, −INE2 (pin 15) voltage).
The FB1 (FB2) terminal voltage is decided for the soft-star t period by the comparison between 1.24 V in an
internal reference voltage and the voltages of the CS1 (CS2) ter minal. The DC/DC conver ter output voltage
rises in proportion to the CS1 (CS2) ter minal voltage as the soft-star t 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)
