The MC33566 Low Dropout Regulator is designed for computer peripheral card applications complying with the instantly available requirements as specified by ACPI objectives. The MC33566 permits glitch-free transitions from "sleep" to "active" system modes and has internal logic circuitry to detect whether the system is being powered from the motherboard main 5.0 V power supply or the 3.3 V aux supply. The MC33566 provides a regulated output voltage of 3.3 V via either an internal low dropout 5.0 V-to-3.3 V voltage regulator or an external P-channel MOSFET, depending on the operating status of the system in which the card is installed. During normal operating mode (5.0 V main supply available) the 3.3 V output is provided from the internal low dropout regulator at an output current of 0.4 A. When the motherboard enters sleep mode, the MC33566 operates from the 3.3 V aux supply and routes the aux current to the output via the external P-channel MOSFET bypass transistor controlled by the drive out pin. As a result, the output voltage provided to the peripheral card remains constant at 3.3 V even during host systems transitions to and from sleep mode.