STM32H725VGH6 / Direct SMPS Supply / VLXSMPS Connection

[SammExa]

Hi,

We are using the STM32H725VGH6 MCU and are plannign to use the internal SMPS for VCORE.

With regards to the connections required for the VLXSMPS pin, I would just like to confirm that a 220pF capacitor is required on this pin in the "Direct SMPS Supply" configuration. It seems unusual to have a capacitor directly on the output of a Buck SMPS. It is also not show in Figure 2 of "AN5419: Getting started with STM32H723/733, STM32H725/735 and STM32H730 Value Line hardware development". However it is recommended in Table 2 & Figure 3.

 

[M_P]

I've been wondering about that too and am very interested in any techincal reasons ST can provide.   220pF directly on the switch node is very unusual.  I observe severe rining on the node between the specified 220pF and 2.2uH inductor (Fresonant = ~7.5 MHz)

 

[M_P]

Linking a similar thread: https://community.st.com/t5/stm32-mcus-products/how-to-prevent-sporadic-ringing-on-vlxsmps-pin-in-quot-direct/m-p/80532

This thread explains the ringing when both switches are off in pulse skipping mode.  However, it does not explain why the 220pF is specified in the first place.  To a power engineer, the 220pF directly on the switch node looks like an error in the schematic.  It would be expected to cause additional switching loss (C*V^2*F) and high peak currents in the SMPS transistors..

[theorykek]

Considering that the QFP tracks inside the chip have a significant conductor length, it can be assumed that the SMPS output is not directly connected to the 220pF capacitor, but rather there is a small inductance between the SMPS switch and the 220pF capacitance.

[SammExa]

Hi @M_P, 

I did see that thread, and it does make sense why the ringing is there, but I agree that I'd like to understand the purpose of the 220pF cap as we flagged it in our review that this was an unusual architecture for a switching node. 

I have lodged a support ticket, so I will update you if I get an answer.

[M_P]

I got this response from ST: 

"I'm confirming that this 220pF is indeed required for designs using LQFP/BGA packages as shown per table 2 in the forum post. 

 

I can see from an end customer's POV how this could be confusing as in Figure 2 there's no explicit connection of a 220pF cap, however the documentation team probably had left that out to avoid confusion for other customers who are not using a LQFP/BGA package. 

I think it would make sense to update the schematic in figure 3 with a note that says 220pF only if device package is of type LQFP/BGA. I'll work on escalating this point with the documentation team. 

 

Furthermore, if you look at our STM32H753I-EVAL's schematic, page 4 where the power supply is configured, you can see a 220pF capacitor connected on the VLXSMPS pin. This board embeds our STM32H753XI which is of package type BGA. "