Low input voltage malfunction fail-safe circuit

MB3788

3. Protection Functions

(1)Timer and latch-type short-circuit protection circuit

The SCP comparator detects the output voltage levels of two comparators to detect an output short circuit. If the output voltage of one comparator increases to 2.1 V, the transistor of the timer circuit is turned off and the short circuit protection capacitor connected externally to the SCP pin (pin 15) starts charging.

The latch circuit turns off the output transistor and simultaneously clears the duty cycle to 0 when the output

voltage level of the comparator does not return to the normal voltage level until the capacitor voltage rises to the base-emitter junction voltage VBE (≈ 0.65 V) of the transistor. (See How to Set Time Constant for Timer & Latch- Type Short-Circuit Protection Circuit.)

When the protection circuit operates, recycle the power to reset the circuit.

(2)Low input voltage malfunction fail-safe circuit

A transient at power-on, or an instantaneous supply voltage drop can cause a control IC malfunction, which may damage the system. The low input voltage malfunction fail-safe circuit detects the internal reference voltage level based on the supply voltage level, resets the latch circuit, turns off the output transistor, clears the duty cycle to 0 and holds the SCP pin (pin 15) at Low level. All circuits are recovered when the supply voltage is greater than the threshold voltage of the fail-safe circuit.