I have a device powered by either USB or a single-cell NiMH, like in the diagram below. The boost converter is resistant to backfeeding, but I need to prevent backfeeding the buck converter when powered by battery alone. What would be the best way? I could add a diode on the 5 V line before the buck converter, but I assume it would still backfeed through GND? Perhaps a MOSFET on the output of the buck?
Update: I'm now considering two solutions:
A). Boost the battery to 3.6 V and then feeding that and USB through the buck converter to reduce both to 3.3 V. The boost converter isn't susceptible to backfeeding, so I should be okay with this. I was hoping to avoid this because of increased losses by boosting the battery higher than necessary, but the boost is 84% efficient and the buck is 92% efficient, so hopefully the losses would be acceptable. This approach has the advantage of leveraging the lower output ripple of the buck converter and ensuring that VCC is exactly the same with either power source.
B) Use the design in the diagram below and put a Schottky on the 5V input. When I asked about this on the TI designers' forum they said that this will prevent backfeeding. The advantage of this is that I get about 9% greater efficiency and battery life, and I can also use a more common 3.3V boost converter instead of a 3.6 V boost.