I have an application with two AA/AAA batteries (so total of 3V or less) powering a unit with a micro controller. I want to design a low cost switch over circuit that can disconnect the battery supply when an external supply is connected with 3.3V. I also want to be able to detect with a micro controller when the external cable is connected supplying 3.3V. I am aware that this can be done with some source selector switch OR controller ICs like LTC4413. But these chips are relatively very expensive and the minimum input volt is only about 2.5V. I would need to go down as much as about 1.8V, as battery can go down to that level. Looking for some ideas how this can be done, thank you.
The most simple solution is to simply Diode-OR both supplies like this:
The downside is that you'll drop 0.7V across a regular silicon diode and about 0.15-0.2V across a Schottky, which may or may not be acceptable to you.
If you want something a little more complex, but will actually turn off any supply from the battery (and probably have less voltage drop when the battery is connected):
The FET should be some sort of low-RDSon FET. Either way you will still need some sort of power conditioning circuit, like a buck-boost so that you can accept voltages higher and lower than your required output voltage. These circuits take care of the power source arbitration.