STM32H757 IC does not boot if VSSMPS connected to VSS of the IC.

[pravali]

Hi,

I am using STM32H757 in one of our projects and operating the IC to power through Internal SMPS only and made the connections as mentioned in the datasheet. The VSSSMPS was connected to VSS of the IC which is a regular practice. With this connection, the STM failed to boot and VFB=VCORE in this case was 50mV always after flashing the FW.

To rectify this, I tried to power the IC using internal LDO by scrapping of VSSSMPS traces (and making it an isolated GND). This rework made the IC boot up.

VSSSMPS and VSS belong to the same IC and should have a common GND. If, it doesn't have a common GND, connecting VSSSMPS to VSS only make the return path of the SMPS longer. But not allowing the IC to boot is very very strange.

Can you help me the explanation?

Also, there is no mention of this in the layout guidelines of the datasheet

Attaching schematic snapshot for reference.

[mƎALLEm]

Hello @pravali and welcome to the community,

According to the schematics you shared, at least VDDLDO is connected to VDDSMPS while in the recommended HW in the reference manual and the STM32H757-Eval schematics, VDDLDO is connected to VFBSMPS. You need to recheck your schematics based on these recommendations:

RM0399 / Figure 22. System supply configurations:

STM32H757-Eval schematics:

 

[pravali]

VDDLDO is connected to VCC_FB as per recommendation.

 

[pravali]

Any response on this? The schematics is as per recommendation.

[mƎALLEm]

Double check again your HW: connections + components and their values and compare them to what was implemented on the STM32H745-DISCO board. Refer to its schematic.

If it is OK then it could be an issue in your PCB.

[pravali]

The hardware is clean. According to the references shared by you of the STM32H757-Eval schematics & STM32H745-DISCO board, the VSSSMPS is isolated GND which I think could be requirement.

I would like to know 

1. If this requirement is captured in any layout guidelines?

2. I understand that it is an Analog GND and is isolated from Digital GND, but if they belong to the same IC, why not be common. How can this halt the booting process?

Can anyone answer these questions?