STM32WB55 Custom Board failing

[BKN636]

Hey guys,

ive build a small custom pcb for the STM32WB55, much like the Usb Dongle (MB1293C, attatched schematic).

Without the µC the board is running fine, its mostly analog stuff + a 16 bit ADC for measuring strain gauges.

Even after having placed the µC and its bypass caps + oscillators, there are no issues.

As soon as i place the inductor (10µH) for its internal switch mode power supply, the board runs for 10-15 minutes. After that the 3V3 supply crashes and my switching regulator (MAX15062A) goes into hiccup mode, without being able to establish a constant supply voltage.

With no fault, powered down, GND to 3V3 rail resistance is about 100Ohms. After fault it goes down to about 5-10 Ohm. And it does not seem to recover after letting the board unpowered for a while.

My suspicion is that theres an internal short circuit inside the µC, apparently due to layout errors, although i tried to keep it close to the reference design of the usb dongle. But i cant seem to find them. Appreciate the help.

[Tesla DeLorean]

Check your netlists for islands, ie where one net becomes multiple due to slight name variations, especially in the ground/power domains.

Work with the ST FAE assigned to your project.

[TDK]

You should really put values in your schematic. Is C19 at least 4.7uF? Have you looked at that power rail to see how it behaves?

Prior to the error, 100 ohms between 3V3 and GND is very low. Where is that power being dissipated? Thermal camera would show it. Might be able to feel it with your hands. Solder bridges are always a possibility.

Mostly just guesses. Can't see any problems in the schematic.

[BKN636]

@Community member​ 

Isolated grounds might be a possibility, the pcb is very cramped and dense (40mm x 18mm), ill look into that. Thanks

Also im not sure what you mean by ST FAE?

@TDK​ I hid the values. Both SMPS caps at the µC Pins 34 & 31 are 4,7µF. All bypass caps are 100nF. Ill try to externally power the board with 3V3, bypassing the switching regulator, and see where it gets hot.

[Tesla DeLorean]

FAE Field Application Engineer / Support Engineer

[BKN636]

Im confused, how would there be someone from ST be assigned to my project?

[BKN636]

Just what i expected happened. I supplied the pcb with a normal external lab supply, current flickered from 100mA to 200mA, µC side at the area around pins 31,32,33,34 (C18=C19=4,7µF,L3=10µH) got pretty warm, then went into a short ciruit and the controller waved goodbye with a heap of smoke. :sad_but_relieved_face:

Now that i checked, the SMPS GND (pin32) is poorly connected, it just goes over the exposed pad, so that the SMPS current has to go an unnecessary long way, which might cause the issue.