EEG board overheats at regulator and fails USB enumeration.

[dunktra]

I'm bringing up a batch of custom 8-channel EEG acquisition boards based on an open, published design. MCU is an STM32L433RCT6, ADC is a TI ADS131M08, intended to be flashed via USB DFU bootloader.

Symptom:
-When the board is connected via USB-C to a host (tested on macOS), it does not enumerate at all. I checked with dfu-util -l, system_profiler, and log stream, none show any sign of a device being detected, not even a partial/failed enumeration attempt.
-Within a few seconds of being powered, a specific region of the board — around the LDO regulator (silkscreen ref U10) and a nearby capacitor (C2) — becomes noticeably hotter than the rest of the board.
-With a multimeter in continuity/diode mode (board unpowered), measuring from V+ to GND near that area produces a continuous continuity beep, which I'm reading as a possible short, but I'm open to being wrong about that interpretation.

Scope:
We have 5 boards from the same production batch. At least one other, independently tested board shows the same heat-and-no-enumeration symptom in the same physical location, so this doesn't look isolated to a single unit.

What I have available to test further:
-Multimeter (continuity/diode/resistance modes)
-Basic soldering iron
-No oscilloscope or bench power supply with current limiting yet

What I'm hoping to learn:
-Is this heat + zero-enumeration pattern consistent with any specific failure mode you've seen before (e.g. reversed/shorted regulator, bad decoupling cap, ESD damage, something else)?
-What's the safest and most reliable next diagnostic step, given what I have on hand?
-Happy to share the schematic PDF, board photos, and netlist if that's useful for anyone taking a look.

[negativ3]

Circuit diagram and photo(s) would be useful.

[Renate]

Well, you said "batch" and "USB" so you could probably use a quick checker (even though this is probably some overall power problem).
You can make this up with either a sa[l]vaged cable or connectors and two LED (no resistors).
If you plug this into a working device the FS (full/high speed) LED will light up, saying, "Please enumerate me".
If nothing lights it's not even asking.

[booscrawl]

I'm bringing up a batch of custom 8-channel EEG acquisition boards based on an open, published design. MCU is an STM32L433RCT6, ADC is a TI ADS131M08, intended to be flashed via USB DFU bootloader.

What I have available to test further:
(...)
-No oscilloscope or bench power supply with current limiting yet

You will need all of these to proceed.

-With a multimeter in continuity/diode mode (board unpowered), measuring from V+ to GND near that area produces a continuous continuity beep, which I'm reading as a possible short, but I'm open to being wrong about that interpretation.

This is pretty unambiguous. Your meter says there's a short. If there's a momentary beep, it could be the capacitors charging up (temporary AC short), but if it's continuous, probably a resistive short.

Do you have ohm mode? How many ohms is the short? Does your multimeter have voltmeter capability or ammeter capability? You could use it as a current-monitoring power supply to diagnose the short if so -- just put it between the power supply and device, but make sure not to blow the internal fuse.

-Is this heat + zero-enumeration pattern consistent with any specific failure mode you've seen before (e.g. reversed/shorted regulator, bad decoupling cap, ESD damage, something else)?

That's actually consistent with a lot of things, and none of them are ESD. I hate to rely on AI here, but my friend at chat.com has some great advice (drawn from this very forum) that I can't disagree with.

Haha, the classic hardware-debugging defense: “Have you considered blaming ESD?” 😄

But USB not enumerating and something overheating makes me suspicious of an actual electrical/design fault before ESD.

A good triage order:

• Stop powering it if something is getting genuinely hot. Especially if an IC, regulator, or USB protection part is hot enough that you can't comfortably touch it.
• Measure the power rails. Check USB 5 V → regulator outputs, and look for an unexpectedly low rail or excessive current draw.
• Check for shorts. With power removed, measure resistance from each supply rail to ground. A very low resistance is a big clue.
• Inspect USB VBUS and GND polarity/routing. A surprising number of “USB won't enumerate” problems start here.
• Check the USB data lines. Look for D+/D− swapped, incorrect pull-up/pull-down configuration, bad routing, missing/incorrect termination components, or an MCU that isn't actually running.
• Verify the MCU clock and reset. If the MCU isn't executing firmware, USB enumeration obviously isn't going to happen.
• Look at the USB device's power consumption. If plugging in USB causes a large current increase, that's particularly informative.
• Only then elevate ESD on the suspect list. ESD can absolutely damage a USB transceiver or MCU without leaving an obvious visual mark—but it doesn't magically explain an otherwise healthy board suddenly drawing excessive power unless something was damaged.

-What's the safest and most reliable next diagnostic step, given what I have on hand?

Invest in test equipment. Review the schematic for obvious errors. Review the datasheets for the regulators more carefully; they're not as forgiving as other ICs.

BTW, ADS131M08 is not rated for EEG and could possibly transmit shocks to you. These might be better: https://www.ti.com/product-category/data-converters/integrated-special-function/medical-afes/biosensing-afes/overview.html