Input undervoltage protection circuit and method

The invention discloses an input undervoltage protection circuit and method. A sampling circuit samples an input voltage and outputs a sampling signal; a control circuit receives the sampling signal and outputs a control signal to a switching circuit and a hysteresis circuit; when an undervoltage occurs, the switching circuit lowers a Vcc pin electrical level of a control IC of the switch power supply; a feedback circuit detects the Vcc pin electrical level and provides a feedback signal to the control circuit to enable the Vcc pin electrical level to be rapidly lowered, and a product is subjected to undervoltage protection; meanwhile, the hysteresis circuit lowers the sampling signal to generate a hysteresis voltage for performing undervoltage protection so as to enable the undervoltage protection for the product to be more reliable; when undervoltage recovers, the switching circuit stops the lowering of the Vcc pin electrical level; the feedback circuit detects the Vcc pin electrical level and provides a feedback signal to the control circuit to enable the switch circuit to rapidly release the Vcc pin electrical level; the Vcc pin electrical level is charged to an IC operating voltage range by an external start circuit, and the product smoothly recovers to operate; and a hiccup phenomenon in the recovery process of the product is eliminated.