Unconnected USB 5V?

[Emil]

Can USB +5V be left unconnected in a circuit which is not USB powered? Or should it be terminated in some way?

[rexxar]

I don't think you need to do anything special. I've left the 5V line disconnected before with no ill effects, but I was moving very little, very slow data. Your mileage may vary.

[Monkeh]

A USB device is required to expose a capacitance on the VBUS pin of its connector of CRPB. This
capacitance shall be greater than CRPB min for voltages on the VBUS pin from 0V to 5.25V,
regardless of whether the USB device is powered or unpowered.

http://compliance.usb.org/index.asp?UpdateFile=Electrical&Format=Standard#59
http://www.usb.org/developers/docs/usb20_docs/usb_20_121314.zip

[katzohki]

A USB device is required to expose a capacitance on the VBUS pin of its connector of CRPB. This
capacitance shall be greater than CRPB min for voltages on the VBUS pin from 0V to 5.25V,
regardless of whether the USB device is powered or unpowered.

http://compliance.usb.org/index.asp?UpdateFile=Electrical&Format=Standard#59
http://www.usb.org/developers/docs/usb20_docs/usb_20_121314.zip

So put a 1uF capacitor on 5V line and leave it at that. Good to know!

[Monkeh]

A USB device is required to expose a capacitance on the VBUS pin of its connector of CRPB. This
capacitance shall be greater than CRPB min for voltages on the VBUS pin from 0V to 5.25V,
regardless of whether the USB device is powered or unpowered.

http://compliance.usb.org/index.asp?UpdateFile=Electrical&Format=Standard#59
http://www.usb.org/developers/docs/usb20_docs/usb_20_121314.zip

So put a 1uF capacitor on 5V line and leave it at that. Good to know!

Minimum 1uF, acros the entire applicable voltage range (0-5.5V). Be mindful of your tolerances and DC bias.

If you're building something USB I do suggest thoroughly reading the specification.