The TPS799-Q1 uses a PMOS pass transistor to achieve low dropout. When (V_IN – V_OUT) is less than the dropout voltage (V_DO), the PMOS pass transistor is in the linear region of operation and the input-to-output resistance is the R_{DS, ON} of the PMOS pass transistor. Because the PMOS transistor behaves like a resistor in dropout, V_DO scales approximately with output current.

As with any linear regulator, PSRR and transient response are degraded as (V_IN – V_OUT) approaches dropout. This effect is illustrated in Figure 6-7 through Figure 6-9 in the Typical Characteristics section.