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This question already has an answer here:

I have such power supply circuit:

Power supply

It can be powered from battery/wall adapter or from USB.

1) When I connect USB, then the 5V from the USB goes to SMPS output. Is that ok? It has EN pin, but I don't know.

2) What happens if a user connects both USB and battery at the same time?
Especially what happens If there's computer plugged to the USB and I connect battery?

3) Do you know any ICs that could multiplex power supply? Something like: I connect USB PS and battery PS to the chip, it detects which one is present (or has control inputs) and send one of them to the output.

I know about LTC4412 but it's too expensive. I would like to avoid using diodes because of voltage drop (first power loss, second 5V won't be 5V any more).

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marked as duplicate by Dmitry Grigoryev, Enric Blanco, Wesley Lee, Voltage Spike, Dave Tweed May 16 '17 at 21:51

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    \$\begingroup\$ is it really critical that the 5v stays 5v? you are feeding it through a DC-DC converter with Vmin=2V \$\endgroup\$ – ratchet freak May 16 '17 at 9:43
  • \$\begingroup\$ Hard to say whether it is critical to be exactly 5V. It may be used by the user for his own purpose - so I cannot say if that matters for him. But if I say that my board provide 5V it would be nice to have those 5V. But even using diodes doesn't answer the question 2). \$\endgroup\$ – zupazt3 May 16 '17 at 10:42
  • \$\begingroup\$ And for the 2V: if the user wants 5V he has to connect at least those 6V. But when battery discharge, to let's say 4V it will give a bit more than 3V on 5V lines, and less than 3V on 3.3V - but uC will still work. \$\endgroup\$ – zupazt3 May 16 '17 at 10:45
  • \$\begingroup\$ If your circuit relies on stable 5V you should use a boost convert. In USB even 4.5V is acceptable, ignoring cable drop. \$\endgroup\$ – Jeroen3 May 16 '17 at 11:30
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    \$\begingroup\$ So essentially you want exactly what LTC4412 does, but you want it cheaper. I'm pretty sure LTC4412 chips would not be on the market if that was easily possible. Also, have you tried to increase the value of R? ;) \$\endgroup\$ – Dmitry Grigoryev May 16 '17 at 11:49
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1) When I connect USB, then the 5V from the USB goes to SMPS output. Is that ok? It has EN pin, but I don't know.

The datasheet for the AOZ1280 says "Do not leave the EN pin floating." In the case where the 6-12V supply is disconnected, that may cause an issue if the USB is powered - you could take a large value resistor from VIN to GND to ensure that EN is not floating, at the expense of increased battery drain and complexity.

2) What happens if a user connects both USB and battery at the same time? Especially what happens If there's computer plugged to the USB and I connect battery?

Any difference in the voltage outputs will result in large currents flowing between the supplies. In general, connecting two supplies together is bad, even if they are nominally the same voltage.

3) Do you know any ICs that could multiplex power supply? Something like: I connect USB PS and battery PS to the chip, it detects which one is present (or has control inputs) and send one of them to the output.

There are many methods to do this, both with ICs and discrete components. This answer seems to summarise it nicely.

I would like to avoid using diodes because of voltage drop (first power loss, second 5V won't be 5V any more).

You can increase the output voltage of the AOZ1280 by changing the feedback resistors. This would at least eliminate the "5V won't be 5V" problem.

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