The ST1534A is designed for applications such as power managed PCI and network interface cards (NICs) where the 3.3V VAUX supply may be required when the 5V supply is shut down. During regular operation, the internal LDO regulator generates 3.3V power for the PCI card from the 5V supply. When the 5V VAUX is available, the IC connects this supply directly to its output using an external P-Channel FET, ensuring uninterrupted 3.3V output even if VIN falls out of specification. When both supplies are available simultaneously, the drive pin DR will be pulled high, turning off the PMOS switch. This product is suitable for applications such as network interface cards, PCMCIA/PCI interface cards, desktop computers, and power supplies with multiple input sources.