I'm designing a circuit that allows one to switch between two different 5V sources, which is then fed into a 3.3V regulator IC. I'm targeting low cost and low board size, and I'll take a solution around $1-$2 in cost. Power supply is an integrated 5V rail on a system and the 5V USB rail for debugging.
The answers there aren't satisfactory for me. The voltage drop on the diode brings the 5V input too low with fluctuations on the 5V rail (ie a lower voltage 5V rail may exceed the bottom limit of my LDO). I'm mainly just wanting a solution that prevents back-feeding of any power source. I also want power to only come from USB if it's plugged in.