This breakout board for Fairchild’s FPF1320 acts as a power multiplexer, allowing you to select which of two power sources is connected to a load while blocking reverse current into either of the sources. It handles up to 1.5 A of current at 1.5 V to 5.5 V and also breaks out a USB Micro-B connector that can be used to supply one of the input power rails.
More technical information at Pololu.
