Anyone know how to disable the SMPS on STM32h757 when only using the LDO?

TAtte.1 · ‎2021-04-26

My board implements this LDO use case from the user manual, no SMPS components and no SMPS circuit:

I need to enable this configuration:

However the device rejects ALL changes to these bits so the default configuration will not change to the LDO supply (or any other supply use case).

Service manual startup sequence has some clues:

Using my multimeter to probe the LDO output, I see 1.0 V but I cannot see 1.2V on the SMPS, because I don't have SMPS components installed on the board as per LDO use case above.

Is the PWR unit waiting for the SMPS 1.2V before permitting changes to the registers?

Any ideas greatly appreciated as I'm not sure if there is a simple work-around.

Service manual found here.

KVan .2 · ‎2022-12-09

I have this same issue. Were able to find a solution?

TAtte.1 · ‎2022-12-10

Reference manual is wrong. SMPS must be enabled or it won't boot. I needed to redesign my pcb to include the smps.

KVan .2 · ‎2022-12-10

Okay thank you for the reply it looks like we might have to do the same.

TAtte.1 · ‎2022-12-10

One idea might be to trick the smps into thinking it has a voltage using a 1.2v voltage divider into VFBSMPS.

I didn't think to try this at the time, but it might be enough to trick the POR into starting the mcu.

Like this:

https://photos.app.goo.gl/NjeLBJWcc7JcXHT47

KVan .2 · ‎2022-12-11

Okay great thank you I might be able to give that a shot and let you know what the result is. Does VDDSMPS also need to be connected to VDD?

TAtte.1 · ‎2022-12-11

I don't know. I think the solution will require testing various connections.

Let me know what you discover.

KVan .2 · ‎2022-12-13

So we were able to get this design to work.

The issue we had was a hardware issue but not with the LDO supply pin configuration. We followed the reference manual. However our boot 0 pin is connected to a button. The button was misplaced causing a short and the device. We fixed this on the board and it all works correctly

KVan .2 · ‎2022-12-13

This was the status of the registers when I first powered on the device with the boot0 pin shorted out. You can see that LDOEN and SDEN pins are enabled in CR3. This should not be the case it should only have LDOEN set. Also notice that the ACTVOSRDY pin is at 0 inside CSR1 register

After fixing the boot0 button this issue was resolved and the CR3 register read the correct configuration and the ACTVOSRDY pin was set high meaning valid