MB3800 Datasheet, PDF (8/21 Pages) Fujitsu Component Limited. – Switching Regulator Controller (Low Voltage)

MB3800
8. How to Set the Time Constant for Soft Start and Short Circuit Detection
8.1 Soft Start
At power on, the capacitor C_PE connected to the SCP pin (pin 2) starts charging. The PWM comparator compares the soft start setting
voltage as a proportion of the voltage at the SCP pin with the sawtooth waveform.
The comparison controls the ON duty of the OUT pin (pin 5), causing the soft start operation. On completion of soft start operation,
the voltage at the SCP pin stays low, the soft start setting voltage stays high, and the circuit enters the output short circuit detection
wait state.
Soft start time (The time until the output ON duty reaches approximately 50%)
t_S [s]  0.35  C_PE [F]
8.2 Short Circuit Protection
If the switching regulator output suddenly drops due to load effect, the error amplifier output (FB pin (pin 8)) is fixed at VOM⁺ and
capacitor C_PE starts charging. When the voltage at the SCP pin reaches approximately 0.8 V, the OUT pin (pin 5) is set low and the
SCP pin (pin 2) stays low.
Once the protection circuit operates, the circuit can be restored by resetting the power supply.
• Short circuit detection time
t_PE [s]  0.8  C_PE [F]
Document Number: 002-08716 Rev. *A
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