MB3887_06 Datasheet, PDF (18/34 Pages) Fujitsu Component Limited. – DC/DC Converter IC for Charging Li-ion battery

MB3887
■ 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 %)
tS (s) =: 0.42 × CS (µF)
= 4.9 V
= 4.2 V
=0V
CS terminal voltage
Comparison with Error Amp block − INE3
voltage.
Soft-start time: ts
VREF
10 µA
10 µA
FB3 15
−INE3 16
CS
22
CS
Error
− ^Amp3
+
+
4.2 V
Q2
UVLO
Soft-start circuit
18