switch between battery and usb power no microcontroller circuit
How about using an alternative circuit like this:



Q4 is a P-mosfet, kept off when there is a voltage in the main supply (USB in this case). When the main supply is disconnected the gate is pulled down and the mosfet conducts and provides output from the battery source.      

The diodes are Schottky type for low Vf and the mosfet should apparently be selected to have a low Vgs-th to turn fully on with the battery voltage level.