STM32H747I-DISCO ground question

[Tycho]

On STM32H747I-DISCO schematic, there are two different grounds. Why is SMPS ground connected to B6, C16 and T12?

[SofLit]

Hello,

I'm checking internally and I'll get back to you if I have an answer.

[SofLit · Accepted solution]

Hello @Tycho ,

I'm getting back to you even after a while (receiving a feedback recently from experts) regarding this topic.

Two points:

1. According to the schematics SMPS_GND and the ground are already connected via SB9
2. According to the board designer the three VSS balls are internally close to some VDDIO33_LDO pins, and consequently, must be connected to SMPS_GND and not VSS as usually, for better signal immunity of LDO and he recommends to follow this rule in your design.

Hope I answered your question.

[Andrew Neil]

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@SofLit wrote:

2. According to the board designer the three VSS balls are internally close to some VDDIO33_LDO pins, and consequently, must be connected to SMPS_GND and not VSS as usually, for better signal immunity of LDO and he recommends to follow this rule in your design..

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Is that information published anywhere ?

[SofLit]

I don't think so.. could be a note to be added in the AN4938: https://www.st.com/resource/en/application_note/an4938-getting-started-with-stm32h74xig-and-stm32h75xig-mcu-hardware-development-stmicroelectronics.pdf

[Tesla DeLorean]

Generally the reason for having TWO nets and bonding them at a single point, is to control the flow of ground/return currents.

With a flood-fill ground plane the electrons will take their own path of least resistance.

With distinct nets, the paths can be controlled and directed, so switching currents around supplies can be isolated and kept away from parts of the circuit that are going to be more sensitive to constant instability.

[Andrew Neil]

Yes,  I definitely think it should go in there - if not in the RM ...

[SofLit]

FYI,

I've escalated the point internally and it was planned  to add a note in a coming revision of AN4938 regarding this point (internal ticket number 184166).

[anengineering]

The balls on the STM32H747I-DISCO schematic make sense as they are next to the VDDLDO balls but it's not so clear on the UFBGA169 package as some VSS balls are close to both VDD and VDDLDO.  It seems they share the VSS balls in two of the 3 instances of VDDLDO balls.

Which pins, if any, do you recommend for the UFBGA169 package?