In the checklist for USB 2.0 compliance there is a requirement for maximum droop at a load inrush current of 7.5 A. What I don't understand is how to test this. I might be blind, but the standard only seems to mention a capacitive load with lower currents (100 mA / 500 mA). Is there a load waveform which shows how the load should actually behave?


Here is what the test procedure says about it:

USB test procedure


I would like to jump in on the question.

For droop testing a specific a specific test fixture is used to simulate the Droop.

You need to load the port under test with the droop portion of the fixture, the droop will vary for USB2.0 and USB3.1 - 100mA/10uF for 2.0 and 150mA/10uF for 3.1

The limits for the Droop test are as follow:

USB 2.0 Bus-Powered hub 100 4.4

USB 2.0 Self-Powered hub 500 4.75

USB 3.0 Self-Powered hub 900 4.45

PC USB 2.0 Host system 500 4.75

PC USB 3.0 Host system 900 4.45

The load waveform will look something like this (Channel 1 is the Droop trigger, Channel 2 is the actual droop which is ~300mv in this picture) : enter image description here

  • \$\begingroup\$ Are you saying that for compliance I need a certified test fixture (I know the one) or is a DIY with the mentioned values in the specification sufficient? \$\endgroup\$
    – Dejvid_no1
    Mar 8 '17 at 18:04
  • \$\begingroup\$ For compliance it is best to use the official fixtures. DIY fixtures varry greatly we ourselves have investigated this, the difference in the transistor used will already affect the Droop (more info @ testusb.com/droop_issue.html) and therefor it is best to use the standardized fixture since allot of small things can impact the results majorly \$\endgroup\$
    – Jan Claes
    Mar 10 '17 at 10:40

I don't what to sound too philosophic, but the answer may be in your own question.

Just reading from the pic in your question:

"For details on testing USB power provision, consult the USB-IF's drop and droop test tescription, which can be downloades from the USB-IF Compliance Program webpage."

USB checklist

It looks to me like the route to go.

  • \$\begingroup\$ Yup, found that too. I added the information that is in the specified test procedure into my question. It doesn't mention the 7.5 A. Maybe it is just a matter of adding capacitance? \$\endgroup\$
    – Dejvid_no1
    Mar 6 '17 at 13:03
  • \$\begingroup\$ Probably, but it may also require a minimum ESR on the capacitor, and/or in series with the wire, along with keeping a load resistor. Without further information I can only guess, sorry. \$\endgroup\$ Mar 6 '17 at 14:49

According to the USB specification Revision 2.0 Aril 27, 2000, chapter "Inrush current limiting" states:

The maximum droop in the hub VBUS is 330 mV


The maximum load (CPRB) that can be placed at the downstream en of a cable is 10 uF in prallel with 44 Ohm. The 10 μF capacitance represents any bypass capacitor directly connected across the VBUS lines in the function plus any capacitive effects visible through the regulator in the device. The 44 Ω resistance represents one unit load of current drawn by the device during connect.

So basically I just need to test the droop with this kind of load and check that the droop isn't too high.


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