STM32U575: VDD 1.8V and USBVDD 3.3V - FAILs (no USB)

[tjaekel]

I want  to run MCU with VDD 1.8V (in order to have all I/O-signals as 1.8V). I changed to use a LQFP64 package which provides a dedicated pin for VDDUSB.

I power VDDUSB with 3.3V (for sure not below 2.7V) but all other VDDs on MCU are 1.8V:

• USB function (FS PHY) FAILS if VDD drops below 1.8V

MCU seems to run with VDD a bit below 1.8V, e.g. 1.794V. But USB is not working anymore!
For USB I need at least 1.804V: then boot is OK, USB comes up and later VDD can drop to 1.793V and still working (but not booting up properly with USB on 1.793V: no USB enumeration anymore).

All my 1.8V ordered and soldered LDOs do not work - because they provide just 1.79xV: USB does not come up.

And the debugger (external ST-Link) becomes flaky with VDD a bit below 1.8V (50% of the tries it does not find the target).

So,

with "off-the-shelf" LDOs (ordered) for 1.8V, even USBVDD is 3.3V - it does not work (with USB, but MCU seems to be running still).

BTW:
With a VDD a bit below 1.8V and VDDUSB 3.3V - I can shortcut very briefly (but not permanently) VDDSMPS with VDD11: then the USB comes up and keeps running, even VDD is a bit below 1.8V (e.g. 1.794V).
It tells me: the SMPS might not start working if below 1.8V and: I would be running anyway "right at the corner of the voltage (specification)" - not very reliable even I could get it to work with finding an LDO providing 1.804 instead of 1.79xV.

Or: Am I doing something wrong?
I am out of ideas why 1.8V VDD and 3.3V VDDUSB should not work, or why the MCU is so "sensitive" on VDD as 1.8V (and a tiny bit below).
BTW: all works fine when I power VDD with greater 1.8V: VDD as 1.8V seems to be right at the edge of spec. I think I am STILL running in the spec. range - but it fails. Is the spec. wrong?

 

[TDK]

Is VDDUSB at/below VDD during power-up?

Can you clarify if you're using SMPS or not?

A schematic would be insightful, if you can provide.

[tjaekel]

I use SMPS (see comments below).
Schematics are attached here.

Comments:

• for SMPS it is important to go with the nominal values, e.g. 10microF on VDDSMPS, total 4.7microF (in sum) on VDD11 (so, each with 2.2microF), keep the external circuit as close as possible to MCU pins (I did)
• USB is failing when VDD drops below 1V8, even VDDUSB is on 3V3 - it seems to happen on STM32U575:
  only on U575 my 1V8 LDOs were failing, on U5A5 all my 1V8 LDOs worked immediately (I have not measured/verified on U5A5 how low VDD can go to have USB working)

All fine for me. I want to go with U5A5 anyway (larger memories) and same board works fine with this version (just U575 seemed to have this issue).

BTW: my VDD is above 1V (1V8 used). And I do not have a specific power-up-scheme on board.

STM32U5A5/STM32U575 MCU board (with LQFP64 package)