MB39A119 Datasheet, PDF (22/53 Pages) Fujitsu Component Limited. – DC/DC Converter IC of Synchronous Rectification for charging Li-ion battery

MB39A119
■ SETTING THE SOFT-START TIME
To prevent rush current at start-up of IC, the soft-start time can be set by connecting soft-start capacitor (CS) to
the CS terminal (pin 17) .
When the CTL terminal (pin 21) is set to “H” level and IC is started (VCC ≥ UVLO threshold voltage) , external
soft-start capacitor (CS) connected to the CS terminal (pin 17) is charged at 10 µA.
ON duty depends PWN comparator output, which compares the FB123 terminal (pin 14) voltage and the trian-
gular wave oscillator output voltage.
During soft-start, FB123 terminal (pin14) voltage increases with sum voltage of CS terminal (pin 17) and diode
voltage. Therefore, the output voltage of the DC/DC converter and current increase can be set by output ON
duty in proportion to rise of the CS terminal (pin 17) voltage. The ON duty is affected by the ramp voltage of
FB123 terminal (pin 14) until an output voltage of one Error Amp reaches the DC/DC converter loop controlled
voltage.
Soft-start time is obtained from the following formula. :
Soft-start time : tS (time to output on duty 80 %)
ts (s) =: 0.13 × CS (µF)
• Soft-start timing chart
CT
FB123
CS
0v
OUT-1
0v
VO
Error Amp3 threshold voltage
0v
IO
0A
CS
FB123
CT
OUT-1
V_O
IO
22