The LM5036 controller contains a three-level under-voltage lockout circuit. When the UVLO pin voltage is below V_SD (0.38-V typical), the controller is in a low current shutdown mode where the functional circuit blocks are not enabled including VCC startup regulator, auxiliary supply and the main half-bridge control logic and gate drive circuitry, etc.

When UVLO pin voltage is above V_SD, the VCC and REF regulators become active.

When the VCC and REF outputs exceed their respective UV thresholds and the input voltage V_IN rises above V_{AUX_UVLO} (15-V typical), the auxiliary supply is enabled.

When UVLO pin voltage rises above V_UVLO (1.25-V typical) and VCC and REF voltage are above their respective UV thresholds, the control logic of the main half-bridge converter is enabled. The soft-start capacitor is released and normal operation begins. An external set-point voltage divider from V_IN to GND can be used to set the minimum operating voltage of the half-bridge converter. The divider must be designed such that the voltage at the UVLO pin is greater than V_UVLO when V_IN enters the desired operating range. UVLO hysteresis is accomplished with an internal current sink I_UVLO (20-μA typical) that is switched on or off into the impedance of the external set-point divider. When the UVLO pin voltage threshold of V_UVLO is exceeded, the current sink is deactivated to quickly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below the V_UVLO threshold, the current sink is enabled causing the voltage at the UVLO pin to quickly fall. See Table 7-1 for more detail on functional modes of LM5036.