Pololu TPS2113A Power Multiplexer Carrier with USB Micro-B ConnectorPololu
The Texas Instruments TPS2113A autoswitching power multiplexer allows you to switch seamlessly between two power sources of 2.8 V to 5.5 V while blocking reverse current into either source. The power multiplexer (or mux) has an adjustable current limit that can be set as high as 2 A and an adjustable switching threshold that helps reduce output voltage droop when switching. Each input channel is controlled by an internal MOSFET switch, so the mux avoids the voltage drop that occurs when diodes are used to OR multiple power supplies together.
By default, the TPS2113A operates in autoswitching mode, which means that it will automatically select the source that has a higher voltage, but the VSNS pin can be used to maintain IN1 as the selected source until it falls below a configurable switching threshold, making IN1 the primary or preferred source. The board ships with USB power connected to IN1, which enables the common application of having a device that can be powered by USB or an external power supply, automatically choosing the appropriate source based on what is connected. USB power can be switched from IN1 to IN2 by cutting a trace and configuring a surface-mount jumper.
Since this board is a carrier for the TPS2113A, we recommend careful reading of the TPS2113A datasheet (1MB pdf).
We carry a similar power multiplexer based on the FPF1320 that is smaller, lower-cost, and works at lower voltages, but it has some limitations that make it not as well suited for automatic USB power-switching applications (e.g. the output voltage can drop to 1 V before the FPF1320 switches to the secondary source).