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Here is the USB to Serial portion of Sparkfun's Stepoko board. Can anyone help me understand why the USB power is isolated from the +5V which is a separate onboard supply powered by an external supply? Nothing except the FT232 is powered by the USB. Hence would it be ok to just leave the F_SMALL fuse and D3 schottky diode unpopulated? Why do you explicitly need to isolate the two supplies?

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If Vcc is higher than +5v + the forward drop of the diode the board will be powered from USB. If +5v is higher than Vcc - the forward drop of the diode then the board will be powered by the on-board supply. By removing the diode and fuse you remove the ability to power the board by the USB port.

You must isolate the two power sources because a USB device is not allowed to drive power onto the USB power lines. Only a host or hub is allowed to do that. The diode prevents the board power from going out through the USB port.

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  • \$\begingroup\$ Thanks. By removing the diode and fuse you remove the ability to power the board by the USB port - I do not want this anyway. The whole board is way too much to be powered just through the USB port. The USB port powers only the FT232RL. \$\endgroup\$ – electrophile Aug 17 '16 at 12:48

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