In a circuit I have three possible power sources: (a) USB with 5V (500mA max), (b) 7.4V+ via a buck converter (3A max) and (c) a direct pin to the 5V rail in the cicuit, 3A max.
The problem is now when source (c) is connected I'd like to disconnect the buck converter (as I fear there might be some problems when it is powered with 5V in reverse direction and open-loop at its input) with a diode or MOSFET, but at 3A the voltage dropt at the diode is too high (the microcontroller needs 4.5V at least). Using different sources or a different microcontroller is not possible.
So how could I do this then? Is it that of a problem when there is about 2.5V at the buck converter in reverse mode, which happens when I use a MOSFET as diode (gate to ground, source to 5V rail and drain to the converter), or can I "disconnect" it complete without voltage drop when there is a power source connected to pin (b) and nothing to pin (c)? I am not really experienced with MOSFETS sadly :/ It should be possible to tune the converter to more than 5V at least, I think up to 5.5V max.
Also, do I need some kind of protection for the USB-connection? Like also disconnecting it with a MOSFET when there is another source connected? Or can the internal diode of those USB-hubs handle my situation?
Thanks for your help.
Oh I totally forgot to mention that I will be using a LT2596 buck converter with the following setup, but with 5V+ setup (or without the two resistors at the top when I choose the 5V fixed version):