BOOTING NUCLEO BOARDS at 4 Volts 150mA max

[rich.g.williams]

When I started working with NUCLEO boards I was very cautious about powering the board up from 5V let alone 12V even powering the board via STLINK USB at 5V (maybe as much as 250mA)

So I did some reading. On my NUCLEO-H5E5ZJ :-

1. The buck regulator ST1S40IPUR  (U16) becomes transparent at under 5V - it just conducts
2. The linear regulator for main 3V3 is LD39200PU33R (U15) and has very low dropout
3.  Most of the STM32H5E5 pins have Absolute Max tolerance rated at 4V
4. If not being used I could put the STLINK into reset via a jumper (less current to board)
5. Avoid initial connection of Ethernet cable or any USB

I tried feeding just under 4V limited to 150mA max into the buck regulator and still got 3.3 Volts out of U15.  The MCU runs fine and I can boot it using USART1. I haven’t learned how to use STLINK just yet :)

For my board with STLINK in reset the Ethernet PHY current about 45mA and total current was 95mA on wakeup and 115mA in bootstrap.

Why do this, well if I short more than 4V to any port pin I risk destroying the STM32H5E5 and in development shorting pins happens.