Do I need to use VDDUSB pin on STM32L4?

[Scott Dev]

Hi

I am designing an application where the STM32L4 is connected to a PCs USB port. The board is always powered up by an external supply input, and not the PCs USB. Can anyone tell me if I have to use the VDDUSB pin on the STM32? I dont have to power anything from the PC connected to the USB port.

Thanks

Scott

[waclawek.jan]

If you intend to use USB, VDDUSB is supposed to be between 3.0V and 3.6V as per DS. If VDD is between these values, you may want to connect VDDUSB to VDD.

JW

[Khouloud GARSI]

Hi @Scott Dev​ ,

The VDDUSB is a dedicated independent power supply for USB. This power supply can be connected either an external independent power supply for USB transceivers (your case) or to VDD. Consequently, the microcontroller can be powered with the minimum specified supply voltage, while an independent power supply 3.3 V can be connected to VDDUSB.

Just note that when the VDDUSB is connected to a separated power supply, it is independent from VDD or VDDA, but it must be the last supply to be provided and the first to be removed.

Khouloud.

[Scott Dev]

Thanks for your replies.

I will connect it directly to my VDD on the processor.

I have another project where I will be using the USB, but also powering the device from it too. As mentioned above "Just note that when the VDDUSB is connected to a separated power supply, it is independent from VDD or VDDA, but it must be the last supply to be provided and the first to be removed" , is there any issues simply using a 5v -> 3.3v regulator from eh 5v line of the input of the usb and powering the VDDUSB and the chips VDD all at the same time?

Thanks

Scott

[waclawek.jan]

> Just note that when the VDDUSB is connected to a separated power supply,

> it is independent from VDD or VDDA, but it must be the last supply to be provided

> and the first to be removed.

This is IMO a much more stringent requirement than what I see e.g. in STM32L476xx datasheet, DS10198 Rev 7, 3.9.1 Power supply schemes, where VDDUSB (as well as other VDDx) is required to be below VDD+0.3V during power-up/down, while VDD is below 1.0V; and completely independent otherwise.

Can you please provide source for that more stringent requirement you posted, Khouloud?

Thanks,

Jan

[Khouloud GARSI]

Hello gentlemen,

Sorry I somehow missed your comments.

@Community member​ : "Can you please provide source for that more stringent requirement you posted, Khouloud?"

-> This is required in order to prevent damaging the chip.

More details could be provided from the datasheet, section "Power supply management".

@Scott Dev​  :  "Is there any issues simply using a 5v -> 3.3v regulator from eh 5v line of the input of the usb and powering the VDDUSB and the chips VDD all at the same time?"

-> Yes, this is possible and also implemented on our eval board.

Khouloud.

[Scott Dev]

Thanks for the diagram.🙂

Scott

[Khouloud GARSI]

You're welcome!

Khouloud.