SNVS124F November 1999 – April 2021 LM2596
Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage. Figure 8-2 shows an undervoltage lockout feature applied to a buck regulator, while Figure 8-3 and Figure 8-4 apply the same feature to an inverting circuit. The circuit in Figure 8-3 features a constant threshold voltage for turnon and turnoff (Zener voltage plus approximately one volt). If hysteresis is required, the circuit in Figure 8-4 has a turnon voltage which is different than the turnoff voltage. The amount of hysteresis is approximately equal to the value of the output voltage. If Zener voltages greater than 25 V are used, an additional 47-kΩ resistor is required from the ON/OFF pin to the ground pin to stay within the 25 V maximum limit of the ON/OFF pin.