Dropout voltage (V_DO) is defined as the input voltage minus the output voltage (V_IN – V_OUT) at the rated output current (I_RATED), where the pass transistor is fully on. I_RATED is the maximum I_OUT listed in the Recommended Operating Conditions table. In dropout operation, 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 value required to maintain 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. Use Equation 1 to calculate the R_DS(ON) of the device.