MB3800 datasheet(8/21 Pages) CYPRESS

Part #	MB3800

Description	Wide supply voltage operating range: 1.8 V to 15 V

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Manufacturer	CYPRESS [Cypress Semiconductor]

Direct Link	http://www.cypress.com

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MB3800

Document Number: 002-08716 Rev. *A

Page 8 of 21

8. How to Set the Time Constant for Soft Start and Short Circuit Detection

8.1 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 [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 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 [F]

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