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.
Measure resistance between +V and Gnd rails. Is your LDO itself overheating? What temp are you reading? Up to about 70 or 80C is not alarming though probably indicating a poor choice in the regulator circuitry.
But, without a schematic it would be hard help much father.
For the future, when I do a first version of a board, I use shorting solder jumpers on the rails to allow isolating different sections. Primarily for characterizing actual current draw of the various subcircuits but can also be useful in tracking down problems like you are having.
Resistance (+V to GND): It is a hard short (0Ω unpowered), not just a diode beep.
LDO temp: Too hot to touch within seconds. Definitely past 70–80°C. I am keeping it powered off.
Schematic: Power section attached.
I noticed EN on the LDO (TLV73333PDBV) is floating. The datasheet says it requires a pull-up to IN.
A floating EN explains the heat and lack of enumeration, but not the unpowered 0Ω short. My theory is that floating EN stressed the internal pass transistor until it failed into a hard short.
Does that theory make sense, or does a 0Ω short point elsewhere?
Love the shorting-jumper idea for isolating subcircuits. Adding that to the next rev!
Check a “virgin” board for a hard short to ground. EN floating will be unpredictable since it has neither pull up or down internally. My best guess is an oscillation. If a virgin board doesn’t have a short to gnd, try a bodge wire EN to Vin. That should at least get you going if the LDO wasn’t destroyed. If a virgin is shorted to ground, time to figure out where the short is, fix it and build a new batch.