MB3800_06 Datasheet, PDF (7/20 Pages) Fujitsu Component Limited. – Switching Regulator Controller (Low Voltage)

MB3800
■ HOW TO SET THE TIME CONSTANT FOR SOFT START AND SHORT CIRCUIT
DETECTION
1. Soft Start
At power on, the capacitor CPE 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%)
tS [s] ≅ 0.35 × CPE [µF]
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 CPE 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
tPE [s] ≅ 0.8 × CPE [µF]
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