Dropout voltage (V_DO) is defined as V_IN – V_OUT at the rated output current (I_RATED), where the pass transistor is fully on. V_IN is the input voltage, V_OUT is the output voltage, and I_RATED is the maximum I_OUT listed in the Recommended Operating Conditions table. At this operating point, the pass transistor is driven fully on. Dropout voltage indirectly specifies a minimum input voltage greater than the nominal programmed output voltage where 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.

For a CMOS regulator, the dropout voltage is determined by the drain-source, on-state resistance (R_DS(ON)) of the pass transistor. Therefore, if the linear regulator operates at less than the rated current, the dropout voltage for that current scales accordingly. The following equation calculates the R_DS(ON) of the device.