If the input voltage is lower than the nominal output voltage plus the specified dropout voltage, the device operates in dropout mode. In this mode, the output voltage tracks the input voltage. In this mode, the transient performance of the device becomes significantly degraded. During this mode, the pass transistor is in the ohmic or triode region, and acts as a switch. Line or load transients in dropout potentially result in large output voltage deviations.

When the device is in a steady dropout state, the pass transistor is driven fully on. This state is defined as when the device is in dropout, directly after being in a normal regulation state, but not during start up. Dropout occurs when V_IN < V_OUT(NOM) + V_DO. When the input voltage returns to a value ≥ V_OUT(NOM) + V_DO, the output voltage potentially overshoots for a short period of time. V_OUT(NOM) is the nominal output voltage and V_DO is the dropout voltage. During dropout exit, the device pulls the pass transistor back into the linear region.