MB3888 Datasheet, PDF (17/30 Pages) Fujitsu Component Limited. – DC/DC Converter IC for Charging Li-ion Battery

MB3888
s METHOD OF SETTING THE SOFT-START TIME
(1) Setting constant voltage mode soft-start
For preventing rush current upon activation of IC, the IC allows soft-start using the capacitor (CS) connected to
the CS terminal (pin 20) .
When CTL terminal (pin 3) 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 (FB2 terminal (pin 6) ) is determined by comparison between the lower voltage of the two
non-inverted input terminals (+INE2 terminal (pin 8) and CS terminal voltage) and inverted input terminal voltage
(−INE2 terminal (pin 7) voltage) . Within the soft-start period (CS terminal voltage < +INE2) , FB2 is determined
by comparison between −INE2 terminal voltage and CS terminal voltage, and DC/DC converter output voltage
goes up proportionately with the increase of CS terminal voltage caused by charging on the soft-start capacitor.
Soft-start time is obtained from the following formula :
Soft-start time : ts (time to output 100 %)
tS (s) =: 0.42 × CS (µF) , at +INE2 = 4.2 V
= 4.9 V
= 4.2 V
(+INE2)
CS terminal voltage
Comparison with Error Amp block −INE2
voltage.
=0V
Soft-start time : ts
VREF
FB2 6
−INE2 ⁷
CS
20
+INE2 ₈
CS
4.2 V
10 µA
10 µA
Error
Amp2
−
+
+
Q2
Soft-start circuit
UVLO
17