MB39A125 Datasheet, PDF (32/64 Pages) Fujitsu Component Limited. – DC/DC Converter IC for Charging Li-ion Battery

MB39A125/126
■ SETTING THE SOFT-START TIME
Soft-start function prevents rush current at start-up of IC when the Soft-start capacitor (Cs) is connected to the
CS terminal (pin 22) . This IC charges external soft-start capacitor (Cs) with 10 µA after CTL terminal (pin 14)
voltage level becomes high and IC starts (when VCC ≥ UVLO threshold voltage) .
Output ON duty depends on PWM comparator, which compares the FB123 terminal (pin 15) voltage with the
triangular wave oscillator output voltage.
During soft start, FB123 terminal (pin 15) voltage increases with sum voltage of CS terminal 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 CS terminal (pin 22) voltage. The ON Duty is affected by the ramp voltage of FB123 terminal
(pin 15) 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
CS
FB123
CT
0V
OUT
0V
OUT
Vo
Error Amp3 threshold voltage
Vo
0V
Io
Io
0A
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