Dropout voltage, V_DO, is defined as the input voltage minus the output voltage (V_IN - V_OUT) where the output voltage falls to 98% of the initial value at the indicated current. See Figure 7-1 for the test waveforms used to measure dropout. The dropout voltage is higher at lower values of V_IN; therefore, dropout voltage is specified across different conditions in Section 6.5.

In dropout, the pass transistor is in the ohmic or triode region of operation, and acts as a switch. The dropout voltage indirectly specifies a minimum input voltage greater than the nominal programmed output voltage at which the output voltage is expected to stay in regulation. If the input voltage falls to less than the nominal output regulation, then the output voltage falls as well.

When V_IN is below 3V, the error amplifier operates with less headroom versus what is present for a V_IN range of 3V to 14V; the reduction in headroom causes higher dropout voltage for V_IN voltages less than 3V(see typical dropout performance graphs Figure 6-1 through Figure 6-6).