STM32N6 Internal SMPS Reference Design

[kevlar]

I've been trying to find examples of the STM32N6 using the internal SMPS however, other than trying to reverse engineer the Nucleo or Discovery boards, there doesn't seem to be much literature available showing this.

Ideally (for cost and board layout reasons), I just want to power the N6 with a single 3.3V supply (plus the 1.8V AON supply) and use the internal SMPS to provide VDDCORE.

Can anyone point me in the right direction, please?

 

[LCE]

> trying to reverse engineer the Nucleo or Discovery boards

IMO that's the best way, because these ST boards are widely sold and working.

But for most boards, I wouldn't "copy" their PCB layout. ;)

[kevlar]

Thanks for your reply.

I did have a look at the power configuration on the Discovery board (I've got the Altium design files) but it's a bit of a chore trying to figure out how the various power rails are linked through to the various power pins of the N6.

Thanks for the hint re the layout. I won't be copying.

[LCE]

> but it's a bit of a chore trying to figure out how the various power rails are linked through to the various power pins of the N6.

Yes, the many options for these boards make it hard to find out what's really required - and it's one reason for not optimal layout.

[LCE]

Getting started with hardware development for STM32N6 MCUs - Application note

Chapter 3.8 shows at least the basic wiring for the internal SMPS.

[kevlar]

Thanks for the link.

I think where I'm getting confused is how many separate regulators I need, taking into account the power sequencing.

As far as I see it, I'd need the following separate supplies:

• 1.8V for VDDA18AON (100mA max)
• 3.3V for VDD (100mA max)

But then, after PWR_ON is asserted:

• 1.8V for VDDSMPS (1.5A ish)

Everything else can be connected through various LC filters?

I think some of the net labels used on the Discovery board and the bloody current measurement circuits are throwing me off a bit. 

[RomainR.]

@kevlar 

If you want an internal SMPS configuration, you do not normally need to manage PWR_ON, it only informs the power-up sequence condition where VDD and VDDA18AON have reached their POR threshold. 
The first condition is to have VDD and VDDA18AON present first. Then if VDDSMPS (1.8V) is present and the external capacitor and inductance are connected, the internal SMPS will generate VDDCore at Startup.

I'm referring to AN5967 in STM32N6 power-up sequence:

Follow the steps below:
1. Apply VDD, VBAT, and VDD18AON.
2. If VDDCORE is supplied from an external SMPS step-down converter, apply VDDCORE when the PWR_ON
signal is high. Otherwise, VDDCORE is internally generated (refer to the section System supply startup and core
domain of the PWR section in the reference manual RM0486).
3. Then, apply the remaining power supplies in whatever order: VDDA18USB, VDD33USBCC, VDDA18CSI, VDDCSI, VDDA18ADC, VREF+, and VDDSMPS.

Concerning the grouping of the different supplies, here are a possible example you can follow:

Note: The domains can be grouped together as follows to simplify the power supply design:
• VDDCSI and VDDCORE.
• VDDA18PLL, VDDIO2, VDDIO3, VDDIO4, VDDIO5, and VDDA18AON.
• VDDA18USB, VDDA18CSI, VDDA18ADC, VREF+.
• VDD33USBCC and VDD33USB.

 

In your case you can design the power scheme below:
1x external DCDC or LDO 1.8V for VDDA18PLL, (optional VDDIO2, VDDIO3, VDDIO4, VDDIO5 if you need to operate at 1.8V), and VDDA18AON.
1x external DCDC or LDO 1.8V for VDDA18USB, VDDA18CSI, VDDA18ADC, VREF+.
1x external DCDC 1.8V dedicated for VDDSMPS (with high power)
1x external DCDC or LDO 3.3V for VDD, VDD33USBCC and VDD33USB and VDDIO2, VDDIO3, VDDIO4, VDDIO5 if you need to operate at 3.3V

Best regards,
Romain,

 

[RomainR.]

Hi @kevlar 

 

For SMPS configuration, could you refer to AN5967 in section 3 Power supplies, and recommended external SMPS components in Table 4. Power supply pins and also section 3.8 SMPS configuration.

Getting started with hardware development for STM32N6 MCUs - Application note

Here some old discussions which can help you:

Solved: Re: STM32N6 minimal hardware design - STMicroelectronics Community

Solved: Re: Building discrete power supply for STM32N6 - STMicroelectronics Community

Best regards,

Romain,

[kevlar]

Thanks @RomainR. The third link seems particularly helpful.

Also, it would be nice if there was a project variant in the Altium design files which showed the components which are not fitted for the internal SMPS, rather than just having a text note. This would be much easier to follow.

[LCE]

1.5A ? Really ?
Or do they mean maximum current spikes / peaks which could be handled by big / enough capacitors?

[kevlar]

This was based on the maximum current on IDDCORE shown in the reference manual with internal SMPS and Tj max of 125 degrees Celcius and all peripherals enabled (1100.0mA). Better to be safe than sorry? Plus, I have other things I want to supply with 1.8V too...

EDIT - the Discovery board has 3A regulators on there for 3.3V and 1.8V...

[kevlar]

Thanks for the comments on this thread. I have enough information now to progress. Cheers!

[LCE]

> Better to be safe than sorry?

Absolutely!

I usually make any voltage supplies for double the maximum current that I expect.

But 1.5A seemed a lot to me, but okay new fast machine...

[kevlar]

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@LCE wrote:

But 1.5A seemed a lot to me, but okay new fast machine...

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And here was me thinking 1.5A wasn't enough! Should be fine for my use case... hopefully