The series pass MOSFET (Q1) is enabled when the input supply voltage (V SYS ) is within the operating range defined by the programmable undervoltage lockout (UVLO) and overvoltage lockout (OVLO) levels. Typically the UVLO level at V SYS is set with a resistor divider (R1-R3) as shown in Figure 30. When V SYS is below the UVLO level, the internal 21-µA current source at UVLO is enabled, the current source at OVLO is off, and Q1 is held off by the 2-mA pulldown current at the GATE pin. As V SYS is increased, raising the voltage at UVLO above 2.5 V, the 21-µA current source at UVLO is switched off, increasing the voltage at UVLO, providing hysteresis for this threshold. With the UVLO pin above 2.5 V, Q1 is switched on by the 16-µA current source at the GATE pin if the insertion time delay has expired (Figure 22). See Application and Implementation for a procedure to calculate the values of the threshold setting resistors (R1-R3). The minimum possible UVLO level at V SYS can be set byconnecting the UVLO pin to VIN. In this case Q1 is enabled when the VIN voltage reaches the POR EN threshold.