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.
Best Answer
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.