STM32H755 PSU Config Modes 2 & 3

[bramble]

Hi, there's something confusing in the schematic for the H755 Nucleo's reference PCB schematic (MB1363).

I don't mind adding that I'm getting a bit tired of confusing and contradictory documentation from ST Micro.

The description of changes when configuring PSU config modes 2 and 3 are as below:

In the highlighted note at lower left it says that to enable Config 3: "Connect VDD_SMPS_IND_OUT to VDD_LDO".

However, looking just above at Config 2 it seems clear that VDD_SMPS_IND_OUT is already connected to VDD_LDO via the node 1V2_MCU which is always connected to both VDD_LDO and VDD_SMPS_IND_OUT.

Can somebody from please confirm how this should work?

My understanding is that the two modes can be summarised as follows (ignoring some capacitor changes):

Mode 2

• VDD_LDO == VDD_SMPS_IND_OUT
• VDBSD == VCAP

Mode 3

• VDD_LDO == VDD_SMPS_IND_OUT
• VFBSD == VDD_LDO

I need a quick answer please because I've a PCB layout needing to go to manufacture.

Thanks

[Fred_ST · Accepted solution]

Hello,
Yes, you are right, the text you highlighted is confusing.
Your understanding of the configuration of the Mode 2 and Mode 3 is correct.
You can also refer to the User Manual UM2408, paragraph 7.4.8 Internal SMPS/LDO configuration which describes in details the different configurations.
Regards

[bramble]

Many thanks Fred. Just to mention that the RM is similarly confusing, it contains two schematics for these two modes, with text that seems to follow the same line that we see in the compete board schematic, but if you follow the wiring it turns out that the delta is indeed quite simple.

I had a concern that I was missing something subtle that would lead to tears when my pcb comes back from fab!