STM32H745II VOS with SMPS supplying LDO

[DTomi.1]

Greetings,

What is the reason for not being able to use the SMPS (or the SMPS supplying LDO) option for VOS0 on STM32H745II?

VOS0 means the highest voltage, so won't the dissipation on the LDO be the highest?

I have been using the PWR_SMPS_1V8_SUPPLIES_LDO option with VOS0 (core running at 480 MHz) and have seen no problem.

Can you please tell me what are the possible risks of using this configuration?

Best regards,

Domagoj

[Foued_KH]

Hello @Domagoj Tomić​ ,

As mentioned in the Table 24. Supply voltage and maximum frequency configuration in the datasheet 12923

VOS0 (power scale 0) is not available when the SMPS step-down converter directly supplies VCORE.

Foued

[DTomi.1]

But I'm currently using this configuration (PWR_SMPS_1V8_SUPPLIES_LDO with PWR_REGULATOR_VOLTAGE_SCALE0) since older CubeMX version allowed it, and it works without any issues so far.

Why is that configuration not allowed, and what are the potential risks if I keep using it?

[DTomi.1]

But I'm currently using this configuration (PWR_SMPS_1V8_SUPPLIES_LDO with PWR_REGULATOR_VOLTAGE_SCALE0) since older CubeMX version allowed it, and it works without any issues so far.

Why is that configuration not allowed, and what are the potential risks if I keep using it?

[Foued_KH]

Try to use the latest version STM32CubeMX 6.8.0 .

[DTomi.1]

My question is regarding the existing FW configured with older CubeMX, what are the possible consequences of using the PWR_SMPS_1V8_SUPPLIES_LDO configuration with PWR_REGULATOR_VOLTAGE_SCALE0 since I already have MCUs shipped with that configuration to customers.

[LCE]

I'm curious about this:

If you find the time, please check with a scope: is the SMPS really active?

And maybe try comparing temperature from MCU-internal sensor, with LDO / SMPS active.

[O'li]

With the hardware layout n°3 ("SMPS supplies LDO" in the above picture), the SMPS seems to be active:

• VDD = 3.3V
• VDDLDO = 1.8V
• VCAP = 1.36V

The internal sensor returns a temperature between 55°C and 60°C.

When disabling the SMPS (firmware configuration = LDO_SUPPLY), VDDLDO and VCORE are set to 0V and the MCU remains stuck (thus I can't get the measurement)

Edit: voltage measurements were swapped

[DTomi.1]

So this is the situation you have:

which is very curious.

How can the output of a linear regulator "V reg" be higher than the input?

EDIT: did you maybe reverse the order of the voltage nets?

I have now measured on my board, VDDLDO is 1.8V and VCAP is 1.36V.

[LCE]

> VOS0 means the highest voltage, so won't the dissipation on the LDO be the highest?

No, but with higher current: yes. ;)

The power the LDO has to dissipate depends on the in-out difference P = (Vin - Vout) * I

So with higher Vout the difference becomes less, so would the power dissipation, if the current stayed constant - which it probably isn't.

[DTomi.1]

Yes you are correct that was a brainfart, I meant higher current.