The CS5231-3 combines a three-terminal linear regulator with circuitry controlling an external PFET transistor thus managing two input supplies. The part provides a 3.3V regulated output either from the main 5V supply or a 3.3V auxiliary that switches on when the 5V supply is not present. This delivers constant, uninterrupted power to the load. The CS5231-3 meets Intel's "Instantly Available" power requirements which follows from the "Advanced Configuration and Power Interface" (ACPI) standards developed by Intel, Microsoft and Toshiba.The CS5231-3 linear regulator provides a fixed 3.3V output at 500mA with an overall accuracy of +/- 2%. The internal NPN-PNP composite pass transistor provides a low dropout voltage and requires less supply current than a straight PNP design. Full protection with both current limit and thermal shutdown is provided.Designed for low reverse current, the IC prevents excessive current from flowing from VOUTto either VINor ground when the regulator input voltage is lower than the output voltage.The CS5231-3 can be used to provide power to an ASIC on a PCI Network Interface Card (NIC). When the system enters a Sleep State and the 5V input drops below 4.4V, the AuxDrv control signal on the CS5231-3 is activated turning on the external PFET. This switches the supply source from the 5V input to the 3.3V input through the PFET, guaranteeing a constant 3.3V output to the ASIC that is "glitch free."The CS5231-3 is available in two package types: the 5-Lead D2PAK (TO263) package and the 8-Lead SOIC 4-Leadfused (DF) package. Other applications include desktop computers, power supplies with multiple input sources and PCMCIA/PCI interface cards.