The TC1267 consists of an LDO, a voltage threshold detector, external switchover logic and gate drive circuitry. It functions as a conventional LDO as long as the voltage on the main supply input (VIN) is above the lower threshold (3.90V typical). Should the voltage on VIN fall below the lower threshold, the LDO is disabled and an external P-channel MOSFET is automatically turned on, connecting the auxiliary supply input to VOUT, and ensuring an uninterrupted 3.3V output. The main supply is automatically selected, if both the main and auxiliary input supplies are present, and transition from one input supply to the other is ensured glitchfree. High integration, automatic secondary supply switchover, Kelvin sensing, and small size make the TC1267 the optimum LDO for PCI 2.0 applications.