I am doing a project that needs USB
communication with my microcontroller.
On the board there is already a 5V power supply. I would like to know if it is correct to connect the VBUS
line from the computer with the 5V line that already exists on my board (put L4). Or should I leave VBUS
floating? (remove L4).
I have no experience with USB development, I know the need for U10, but how I should feed U10 pin 5 to me is still dubious.
Best Answer
If your board has it's own power, the VBUS from upstream connector should NOT be used as power source and should NOT be connected to the +5V rail. It will cause unpredictable conflict between your on-board power source and USB host-supplied VBUS.
However, your device must have a circuitry (typically a GPIO input with proper level translation and additional ESD protection) to sense the VBUS presence. This function is defined in Section 7.1.5.1 of USB 2.0 Specifications,
and further explained in Section 7.2.1, p.171:
If you have a concern about pin5 on ESD protector U10, then it would be the best to keep this pin at 3.3V form some internal rail, it will provide somewhat better level of protection.
There are more answers on this topic, like here.