1.8V VDD and 3.3V VUSB

[koendv]

Some STM32's have VDD and VDDUSB. Setting VDD to 1.8V and VDDUSB to 3.3V, the GPIOs work at 1.8 V, and USB keeps working (the USB transceiver needs 3.3V). Each time, there is a note that VDDUSB has to be the last power supply to be active when powering up, and VDDUSB has to be the first power supply to disappear when powering down. Examples:

• STM32F412 datasheet, Figure 6. VDDUSB connected to an external independent power supply
• STM32WB55 datasheet, Figure 6. Power-up/down sequence

Does ST have any suggestions or schematic for sequencing power supplies that supply 1.8V and 3.3V, and comply with the requirements for STM32 mcu's? TPS70351? The F412 NUCLEO board has VDD and VDDUSB connected, so that is no help.

[Pierre_Paris]

Hello @koendv ,

Thank you for your question.

In the DS, it's written : "VDDUSB can be connected either to VDD or an external independent power supply (3.0
to 3.6 V) for USB transceivers.".

Here are some information :

- You can find here eDesignSuite, a comprehensive set of easy-to-use design-aid for power supply design tool. You enter your input and output voltages and components with schematics are suggested for your application.

- You can also read this article about Power Management ICs (PMIC) especially the DC-DC switching converters part.

Best Regards,

Pierre

[koendv]

Thank you for your reply. Yes, this will get me 3.3V and 1.8V.
But the delicate part is the power sequencing. Suppose the f412 is powered from usb.
When plugging the device in usb, the 3.3V should wait until 1.8V is active. Fine.
And when someone unplugs this device from usb, the 3.3V should shut down first, then the 1.8V.
Does ST have a schematic for this? It's in the datasheet, so I suppose someone somewhere built the circuit.

[Pierre_Paris]

Hello @koendv ,

Suppose the f412 is powered from USB.

In your application, that means VDD & VDDUSB come from the same VBUS power supply, right ? 

You can then use regulators LDO to drop your voltage to 1.8 & 3.3. If the voltage arrive at the same time during a transition period, it's not a problem. The RM mentioned " When the three power supplies are shut down, if VDD_USB remains active for a short period of time and VDDA/VDDIO fall below the functional range, the device will not be damaged."

Another solution could be adding a transistor with a pull down resistor driven by a GPIO.

Best Regards,

Pierre

[koendv]

Thank you. That fully answers my question.