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I'm working on a prototype board which has multiple USB device connectors (e.g. the one for a microcontroller thing I'm communicating with, and a second one for JTAG).

Now I wonder how I should treat the VCC line, because I'd like to power the board from either USB connector.

Sure, I can put a Schottky diode in line with each VCC and call it a day. I don't want the voltage drop though. I power some things from 5V and on some USB hosts I already see voltages as low as 4.5V instead of 5V. Loosing another 0.3V or so would be critical.

Since I'm going to power the device from a single USB Hub anyway, would it be okay to just connect the two VCC lines? The only other option I can see now is to forget about USB power, leave VCC open and use an external power supply.

Nils Pipenbrinck
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2 Answers2

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Directly connecting the power from multiple USB hosts is not a good idea. If you want well regulated power voltage, you'll need to have a separate power supply for that purpose anyway since USB voltage can vary quite a bit. If I remember right, the lowest is around 4.3 V, and the highest over 5.

Put a Schottky diode in series with each USB power feed, then regulate from there. Even with a diode drop, there is still room for a LDO to make a clean and regulated 3.3 V. If you really need 5 V (do you really need 5 V?), use a small boost converter to make reliable 5 V from the USB power after the diodes. You'd have to do that anyway if you want reliable 5 V.

Olin Lathrop
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  • You're right, in the end I'll don't get around a proper 5V power supply anyways. And yes, I really need the 5V. Three times as much even. I currently generate 9V and -4V (roughly) from the 5V USB voltage for several analog ICs. – Nils Pipenbrinck Sep 02 '15 at 10:37
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No, it is not a good idea to just connect all of the 5.0V lines from the USB connectors. Depending on the actual voltage levels of the USB sources, you may end up with current flowing into one of the USB hosts which is not good.

Look at the answers to this question for some options to combine power sources with minimal voltage loss.

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DoxyLover
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