MB3888 Datasheet, PDF (18/30 Pages) Fujitsu Component Limited. – DC/DC Converter IC for Charging Li-ion Battery

MB3888
(2) Setting constant current 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 (FB1 terminal (pin 15) ) is determined by comparison between the lower voltage of the two
non-inverted input terminals (+INE1 terminal (pin 13) and CS terminal voltage) and inverted input terminal voltage
(−INE1 terminal (pin 14) voltage) . Within the soft-start period (CS terminal voltage < +INE1) , FB1 is determined
by comparison between −INE1 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) =: +INE2 / 10 (µA) × CS (µF)
= 4.9 V
= +INE2
CS terminal voltage
Comparison with Error Amp block −INE1
voltage.
=0V
Soft-start time : tS
VREF
FB1 15
−INE1 ¹⁴
CS
20
+INE1 ₁₃
CS
10 µA
Q2
10 µA
Error
Amp1
−
+
+
Soft-start circuit
UVLO
18