Using SMPS in STM32H745 to power core and IO

[cos]

The SMPS in STM32H745IIK can optionally supply a fixed 1.8V to the internal LDO and potentially some external circuitry. In my case, I have a board with a 3.3V supply, and I want to utilize the built-in SMPS to provide a 1.8V supply and 1.8V IO levels for the STM32 MCU and a few sensors. However, based on my understanding of the documentation, it seems that supplying VDD (and consequently IO) of the STM32 with 1.8V directly from the internal SMPS is not feasible. This is because VDD must already be supplied to configure the SMPS during startup. Is my conclusion correct?

[ELABI.1]

Hi @cos ,

Do you mean Direct SMPS supply by internal SMPS? Could you please explain more your schematic requirement?  

For more information and conditions about SMPS, I recommend you take a look at Getting started with STM32H74xI/G and STM32H75xI/G MCU hardware development - Application note and Datasheet DS12923 specifically section 3.5 Power supply management.

I hope this help you.

ELABI.1

[cos]

Thank you for reply. I'm referring to the supply scheme, described in AN4938, 2.1.5, p.2 - External supply mode. Where internal SMPS is supplying fixed voltage (2.5V or 1.8V) to an internal LDO and optional external circuits. The schematic for this supply scheme is in RM0399 Figure 22, option 4. So my question is, while I could supply some external components (like low power sensors) from the internal SMPS with fixed output of 1.8V, I cannot supply VDD of the STM32H7 itself to make use of 1.8V IO between MCU and sensors?

[ELABI.1]

Hello @cos,

In fact, the SMPS is working (as GPIO) on the main supply of the product, on this product there is no split IO ring allowing an independent supply at a lower voltage.

In your case, the product is supplied by 3.3V, which implies that the GPIO are working at 3.3V. The SMPS is then able to provide an alternative voltage of 1.8V to supply an external component and/or the internal LDO.

If you need all the GPIO to work at 1.8V, you can supply the full chip with 1.8v and the internal SMPS (preferred solution) or LDO could be used to supply the VCore.

Thank you.

ELABI.1