MB39A132QN-G-ERE1 Datasheet, PDF (26/56 Pages) Fujitsu Component Limited. – ASSP For Power Management Applications (Rechargeable Battery) Synchronous Rectification DC/DC Converter IC for Charging Li-ion Battery

MB39A132
■ SETTING THE SOFT-START TIME
To prevent rush current at start-up of IC, the soft-start time can be set by connecting a soft-start capacitor
(CS) to the CS pin (pin 19). When the CTL1 pin (pin 23) and the CTL2 pin (pin 32) are set to “H” level and
the IC is started (Vcc 3 ≥ UVLO threshold voltage), the external capacitor (Cs) for soft-start (CS) connected
to the CS pin is charged at 10 μA.
The output ON duty depends on the result of comparison done by the PWM comparator among the COMP1
pin (pin 8) voltage, the COMP2 pin (pin15) voltage, the COMP3 pin (pin16) voltage and the triangular wave
oscillator output voltage (CT). During soft-start, the COMP1 pin, the COMP2 pin, and the COMP3 pin voltages
are clamped so that the voltages of those three pins will not exceed the CS pin voltage. Therefore, the output
voltage of the DC/DC converter and current increase can be set by the output ON duty in proportion to rise
of the CS pin voltage.
The ON duty is affected by the ramp voltage of the COMP1 pin, the COMP2 pin, and the COMP3 pin until
the output voltage of one of the three Error Amp reaches the DC/DC converter loop control voltage.
Soft-start time is obtained from the following formula.
Soft-start time (time for the output ON duty to reach 80%): ts(s) =: 0.23 × Cs (μF)
CT
COMP1 to
COMP3
CS
0V
OUT1
0V
Error Amp3 threshold voltage
Vo
0V
Io
0A
CS
COMP1 to
COMP3
CT
OUT1
Vo
Io
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DS04–27265–3E