USB 5VDC specs

The voltage drop budget is determined from the following:

=B7 The voltage supplied by high-powered hub ports is 4.75 V to 5.25 V.

=B7 The voltage supplied by low-powered hub ports is 4.4 V to 5.25 V.

=B7 Bus-powered hubs can have a maximum drop of 350 mV from their cable plug (where they attach to a source of power) to their output port connectors (where they supply power).

=B7 The maximum voltage drop (for detachable cables) between the A-series plug and B-series plug on VBUS is 125 mV (VBUSD).

=B7 The maximum voltage drop for all cables between upstream and downstream on GND is 125 mV (VGNDD).

=B7 All hubs and functions must be able to provide configuration information with as little as 4.40 V at the connector end of their upstream cables. Only low-power functions need to be operational with this minimum voltage.

=B7 Functions drawing more than one unit load must operate with a 4.75 V minimum input voltage at the connector end of their upstream cables.

You can download the USB spec at :

Cheers

PeteS