7.3.2 UVLO

The LM5119 contains a dual-level undervoltage lockout (UVLO) circuit. When the UVLO pin is less than 0.4 V, the LM5119 is in shutdown mode. The shutdown comparator provides 100 mV of hysteresis to avoid chatter during transitions. When the UVLO pin voltage is greater than 0.4 V but less than 1.25 V, the controller is in standby mode. In the standby mode the VCC bias regulators are active but the controller outputs are disabled. This feature allows the UVLO pin to be used as a remote enable or disable function. When the VCC outputs exceed their respective undervoltage thresholds (4.9 V) and the UVLO pin voltage is greater than 1.25 V, the outputs are enabled and normal operation begins.

An external set-point voltage divider from the VIN to GND is used to set the minimum VIN operating voltage of the regulator. The divider must be designed such that the voltage at the UVLO pin is greater than 1.25 V when the input voltage is in the desired operating range. UVLO hysteresis is accomplished with an internal 20-μA current source that is switched on or off into the impedance of the set-point divider. When the UVLO pin voltage exceeds 1.25-V threshold, the current source is activated to quickly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below the 1.25-V threshold, the current source is turned off causing the voltage at the UVLO pin to quickly fall. The UVLO pin must not be left floating.