Generally speaking, the dropout voltage often refers to the minimum voltage difference between the input and output voltage (V_DO = V_IN – V_OUT) that is required for regulation. When V_IN drops below the required V_DO for the given load current, the device functions as a resistive switch and does not regulate output voltage. Dropout voltage is proportional to the output current because the device is operating as a resistive switch, as shown in Figure 8-11.

Figure 8-11 Dropout Voltage versus Output Current

Dropout voltage is affected by the drive strength for the gate of the pass element, which is nonlinear with respect to V_IN on this device because of the internal charge pump. Dropout voltage increases exponentially when the input voltage nears its maximum operating voltage because the charge pump multiplies the input voltage by a factor of 4 and then is internally clamped to 8 V.