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In my circuit I'm using a TPS78633DCQ low-dropout regulator to regulate my 5 volts power input down to 3.3 volts. However, when powering up the circuit board for the first time today and measuring the voltages, I was getting 5 volts on the 3.3 volt rail.

There was no smoke or anything — I was watching quite carefully for this during the first power-on — and none of the chips on the board were getting hot when I felt around with my fingers; I was also very careful to check for shorts not just to ground but from each rail to every other rail before turning it on, and there weren't any.

On probing it with my oscilloscope and examining it while turning it on, it looks like the regulator isn't actually turning on at all until 10-12 seconds after first applying power, and when it does it jumps all the way to 5 volts:

scope image showing 3.3 volt rail jump to 5 volts 12.5 seconds after applying power (yellow is 5 volt rail, pink is 3.3 volt rail)

Note that this is on an assembled printed circuit board, so I don't believe it's possible this could be a grounding problem; the ground and thermal pins both have vias directly to the internal ground plane as close by as I could manage.

photo of assembled board photo of probing setup

This is the power supply section of my schematic: power supply schematic

U17 is the 3.3 volt regulator that appears to be behaving incorrectly; initially I suspected it may have been assembled with an incorrect chip, but the chip on the board at the very least has the correct markings:

linear regulator chip showing part number

Here's my full schematic, pcb layers, and EDA project, for reference:

Full Schematic (pdf)
PCB Layers (pdf)
EasyEDA Project

Note that other 1.8 and 1.1 volt rails did come up to their correct voltages after that, so those regulators are at least functional (were able to regulate down the 5 volts rather than the 3.3 they ought to have been getting).

However having 5 volts on the 3.3 volt rail did exceed the voltage rating on the TS3USB221 muxes (datasheet says 4.6V absolute max for VCC) and TUSB4041 hub chips (datasheet says 3.8V absolute max for VDD33), so it most likely has damaged this board. Hopefully I can fix the problem with the regulator on the other boards before powering those ones up.

What is going on here? Did I not connect the voltage regulator (enable pin?) correctly, or something? Were my boards assembled with a wrong/counterfeit chip despite the markings? I have never seen a behavior like this before, and I have essentially no idea what to actually do about it.

For the moment, I can remove the regulator from my other boards and feed their 3.3 volt rails with a bench supply (in order to test the other features of these boards), but I don't even have any concept about how I would redesign the circuit to make this not happen.

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Short answer : The TPS78633 NR pin is incorrectly connected to 3.3V output, and it is causing the issue.

This overrides the internal Vref of 1.25V with the output voltage, and it tries to set the output voltage high enough so the divided feedback matches the reference voltage, so basically, the regulator just fully passes input voltage to output.

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  • \$\begingroup\$ Note to OP that although the symbol pin is labelled NR/FB, per datasheet the FB bit applies only to the adjustable version at which point the pin should connect via a resistive divider, and the NR applies only to the fixed versions which should be left unconnected, or bypassed to ground via a low value capacitor. \$\endgroup\$ – Tom Carpenter Aug 2 at 18:05
  • \$\begingroup\$ Ah, ok that makes sense. I'd based my power supply on the one from TI's reference schematic for the USB hub chip, I hadn't realized I'd spec'd a TPS78633 instead of the TPS7A4533 they had, or that it had an incompatible pinout. In any case, I've snipped that leg and it's working perfectly now, so thank you! Here's hoping the chips on the first board survived their brief overvolting. \$\endgroup\$ – AJMansfield Aug 2 at 18:37
  • \$\begingroup\$ Also, why does it take 12 seconds for the output to start going to 5 volts? Why doesn't it go up to 5 volts immediately? \$\endgroup\$ – AJMansfield Aug 2 at 18:46
  • \$\begingroup\$ That is hard to say, because, the regulator is not anyway being used in the intended way. \$\endgroup\$ – Justme Aug 2 at 18:56

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