The TPS4021x is designed to operate with input voltages above 4.5 V. The typical VDD UVLO threshold is 4.25 V and the device can operate at input voltages down to the UVLO voltage. At input voltages below the actual UVLO voltage, the device will not switch. When VVDD passes the UVLO threshold the device will become active. Switching is enabled and the soft-start sequence is initiated. The TPS4021x will ramp up the output voltage at the rate determined by the external capacitor at the SS pin.