Is there any design specification in the USB forum that states what amount of back powering current from a device to a hub is allowed? I understand that it may be zero but I also can't seem to find such a condition anywhere in the documentation.
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What version of USB? – Jeroen3 Feb 21 '20 at 09:25
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USB 2.0, But if there is info available on newer versions, please share them. – anxiousPI Feb 21 '20 at 09:27
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Tests for back driving voltage are described in “USB-IF Full and Low Speed Electrical and Interoperability Compliance Test Procedure" document. The test uses the following fixture:
Voltages on 15k loads should not exceed 400 mV.
Ale..chenski
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I think this is helpful, but I am mostly concerned about inrush current coming from a power supply that makes the device use its voltage instead of the hub's voltage. I'm not very sure back voltage can help with identifying that. – anxiousPI Feb 24 '20 at 11:27
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@anxiousPI, this is not just "helpful", but answers your question precisely. Your concern about "inrush current coming from a power supply" makes no sense. Please try to learn exact terminology before asking a technical question. – Ale..chenski Feb 26 '20 at 04:51
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Do you know what the analogous test would be for USB 3.0? Should we also do the same tests on the superspeed pins? – interoception Jan 10 '22 at 15:12
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For anyone interested interoception's question was answered here: https://electronics.stackexchange.com/questions/603765/how-do-you-do-back-voltage-testing-on-usb-3-0 – tom r. Jul 26 '22 at 05:14