SNVSB97A December 2018 – September 2021 LM34936-Q1
The LM34936-Q1 has a dual function enable and undervoltage lockout (UVLO) circuit. The EN/UVLO pin has three distinct voltage ranges: shutdown, standby, and operating (see Section 7.4.1). When the EN/UVLO pin is below the standby threshold VEN(STBY), the converter is held in a low power Shutdown mode. When EN/UVLO voltage is greater than the standby threshold VEN(STBY) but less than the operating threshold VEN(OP), the internal bias rails and the VCC regulator are enabled but the soft-start (SS) pin is held low and the PWM controller is disabled. A pullup current IEN(STBY) is sourced out of the EN/UVLO pin in Standby mode to provide hysteresis between Shutdown mode and Standby mode. When EN/UVLO is greater than the operating threshold VEN(OP) and VCC is above the undervoltage threshold VUV(VCC), the controller starts operation. A hysteresis current ΔIHYS(OP) is sourced out of the EN/UVLO pin when the EN/UVLO input exceeds the operating threshold to provide hysteresis that prevents on/off chattering in the presence of noise with a slowly changing input voltage.
The hysteresis between the UVLO turn-on threshold and turn-off threshold is set by the upper resistor in the EN/UVLO resistor divider and is given by: