NRF52-DK faulting with external 3v supply (twice)

[snoopy20]

I've a prototype board which I need to externally program. The prototype board supplies it's own 3v supply to the chip (working fine) via a 5v supply which is current limited to no more than 100mA.

The nRF52-DK is powered by USB. I blew one yesterday (heat near the battery area) and a second that arrived this morning has just faulted.

I connect GND, VDD-2-VTG, IO and CLK. This causes the fault, and also to blow the NRF52810 on the prototype board.

If I use the boards VDD-nRF 3v supply it works fine.

[snoopy20]

I left the second DK for a while and went back to it. LED5 slowly ramped up and eventually the other LED's come on and the board is back to normal. Now it works fine on each plug/unplug. I suspect a chip was partially damaged but not enough as per the first board which is dead (and heating up).

[snoopy20]

Actually the external pins on this are damaged. Third board is working well!