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Selecting input power - external, usb or batteries


I'm modifying an existing design that has 3 different power inputs:

Ext power - ~12V

Batteries - 6 x AA

USB - 5V

The design currently just diode 'OR's' the supplies together before the main switching regulator.

The problem is that when there is no external power, but there are batteries and USB, the current is drawn from the batteries as they are a higher voltage. I'd really like to be able to isolate the batteries completely when on USB power.

I've been looking at high side and low side switches etc, but have yet to come up with a good solution.

I don't want to redesign the power supply - too much risk, and I'm limited for board space. I also don't want to introduce any significant losses that will limit run time on the batteries.

Any good ideas?


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