8.1.3 Dropout Voltage

The TPS720 uses a NMOS pass transistor to achieve low dropout. When (V_IN – V_OUT) is less than the dropout voltage (V_DO), the NMOS pass device is in the linear region of operation and the input-to-output resistance is the R_DS(ON) of the NMOS pass element. V_DO approximately scales with output current because the NMOS device behaves as a resistor in dropout.

As with any linear regulator, PSRR and transient response are degraded as (V_IN – V_OUT) approaches dropout. This effect is shown in Figure 19.