An undervoltage lockout pin is provided to disable the regulator without entering shutdown. If the UVLO pin is pulled below 1.215 V, the regulator enters a standby mode of operation with the soft-start capacitor discharged and outputs disabled, but with the VCC regulator running. If the UVLO input is pulled above 1.215 V, the controller will resume normal operation. A voltage divider from input to ground can be used to set a VIN threshold to disable the supply in brown-out conditions or for low input faults. The UVLO pin has a 5-μA internal pull up current that allows this pin to left open if the input undervoltage lockout function is not needed. For applications which require fast on/off cycling, the UVLO pin with an open collector control signal can be used to ensure proper start-up sequencing.

The UVLO pin is also used to implement a “hiccup” current limit. If a current limit fault exists for more than 256 consecutive clock cycles, the UVLO pin will be internally pulled down to 200 mV and then released, and a new SS cycle initiated. A capacitor to ground connected to the UVLO pin will set the timing for hiccup mode current limit. When this feature is used in conjunction with the voltage divider, a diode across the top resistor can be used to discharge the capacitor in the event of an input undervoltage condition. There is a 5-μs filter at the input to the fault comparator. At higher switching frequency (greater than approximately 250 kHz) the hiccup timer can be disabled if the fault capacitor is not used.