Is it really required that VDD33USB is lower than VDD during power up/down?

[BNord.1]

Hello ST Community,

I'm planning to use the USB communication with a STM32H743ZI. Then, I will have to use an external power supply, since it doesn't have VDD50USB input. Furtherore, I will power this MCU with a 3.3Vcc. power supply.

However, I read in the AN4938 (Getting started with STM32H74xI/G and STM32H75xI/G hardware development) the following statement:

"During the power-on and power-down phases (VDD < VDD minimum value),VDD33USB should always be lower than VDD".

If I connect directly the VDD33USB to the VDD, I won't be compliant with this recommendation. What would be the consequences during the power on/off? Is there a risk that the USB communication fails?

In any case, would there be a power management IC that would you suggest to use if I wanted to be compliant with this recommendation?

Thanks a lot for your help.

[TDK · Accepted solution]

I think that's only if it's connected to something other than the main power supply at 3.3V. It would be odd otherwise.

[BNord.1]

Totally agree! I haven't understood it that way: I thought it meant "when the internal regulator supply was not availabe"...

Thanks for enlightening me 🙂