PWR Configuration for STM32H755ZIT3 (LDO-only mode)

[RIVR]

I am currently performing the bring-up of a custom PCBA using the STM32H755ZIT3.
My design is intended to use the Internal LDO only, bypassing the SMPS to reduce component count (i.e. one less inductor in my layout).

Here's few details about the MCU power section of my SCH (also, please check attached capture):

• SMPS Pins: As per the Reference Manual for "LDO-only" mode, I have grounded VFBBSMPS, VSSSMPS and VDDSMPS, whereas VLXSMPS is floating.

• VCORE/VCAP: I have placed the required 2.2uF decoupling capacitors on VCAP_1, VCAP_2, and VCAP_3. No external inductor is present on the layout.

What I'm observing is that at power-on, VCORE remains at 0V. Consequently, the target is not detected by the ST-LINK. If I instead "jumpstart" the board by applying an external 1.2V to the VCAP nodes, the MCU becomes reachable via SWD, and I can successfully modify registers.

I suspect a conflict between my hardware layout and the factory default reset value of the PWR_CR3 register (see attached abstract of RM0399). Looking at the documentation, if the SMPS is enabled by default, the power-on sequence hangs because the SMPS cannot regulate without the missing inductor.

My question for you is: is there an easier way to have the MCU wake up with LDO-only config than externally forcing 1.2V on VCAP? Am I really forced to have an inductor in my layout?

Thank you,
NB

[TDK]

What you've done should work. If not, there is an error somewhere, but it doesn't appear to be anywhere that you've shown.

Could be a poorly soldered chip, bad connection, misoriented part, design issue, etc.

[mƎALLEm · Accepted solution]

Hello @RIVR and welcome to the ST community,

You may need to refer to the AN4938 "Getting started with STM32H74xI/G and STM32H75xI/G MCU hardware development":

A reference schematic:

So,

1- I don't seen any of the decoupling capacitors on VDD in your schematic

2- VCAP pins need only 2 x 2.2uF. Not one cap per VCAP pin.

[RIVR]

Hello, and thank you both for addressing my ticket.

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@mƎALLEm wrote:

Hello @RIVR and welcome to the ST community,

You may need to refer to the AN4938 "Getting started with STM32H74xI/G and STM32H75xI/G MCU hardware development":

A reference schematic:

So,

1- I don't seen any of the decoupling capacitors on VDD in your schematic

2- VCAP pins need only 2 x 2.2uF. Not one cap per VCAP pin.

---

Coming to your points:

1- My bad, I didn't realize I cropped off the decoupling caps from the snapshot. Decoupling caps are actually there, as per attached capture.
2- Ah, noted, I overlooked this. Will save one 2.2uF in my next re-spin then, thanks!


Anyways, small piece of update: I tried bringing-up a blank PCBA unit to make sure there was no board-specific issue with my first setup. Turns out the first PCBA was indeed damaged (somehow), and now I can reliably see 1.00V on VCORE on other nominally identical units as soon as I power them up.
I'll maybe try and replace the MCU on the defective one, but I would say the issue is now solved!

Thank you all for the valuable support, truly appreciated!
Best,
NB