3
\$\begingroup\$

I am working on a device that is powered from a USB host and has a downstream-facing USB type A receptacle. VBUS on the downstream port is connected directly to VBUS on the upstream port, and 3.3V power for the device comes from an LDO with its input connected to VBUS. I chose the TPS79333 because I had a bunch in my parts collection already and have used it before in other designs. The recommended max input voltage is 5.5V (6V absolute maximum).

I found that plugging one USB hub in particular (I wasn't able to reproduce this with any other devices) into the downstream USB port will always cause the LDO to latch up, causing its output to become shorted to the input and it to draw about 400mA from input to ground. The LDO recovers fully after removing power, and the everything works as expected if I connect the USB hub before applying power. I found two different solutions that both solved the problem: 1) connecting the USB hub through a 2 meter long extension cable and 2) adding an additional 10uF decoupling capacitor on the input of the LDO.

Was my original design flawed, or is this USB hub just poorly designed? Is adding additional capacitance to VBUS a reliable solution to this problem, or is there a better way to solve it? Should I be using an LDO with a higher maximum input voltage?

enter image description here

\$\endgroup\$
1
  • \$\begingroup\$ are the USB data lines connected? \$\endgroup\$
    – jsotola
    Commented Nov 15, 2022 at 22:59

1 Answer 1

2
\$\begingroup\$

During USB hotplugging, you basically connect the capacitors of output port to capacitors of input port, with wire that has resistance and inductance.

It means that the 5v can momentarily drop quite low, especially if the powered side capacitance is too low compared to capacitance of the unpowered side.

What likely happens is that the voltage dips below 3.3V, and in the list of things to allow and not allow, the case where regulator input becomes much less than output is definitely in the do not allow list.

Likely the cable adds enough resistance and inductance that hot-plugging does not dip the regulator input too low.

You can easily monitor for that scenario with an oscilloscope.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.