STM32L4 - Connection of VddUSB Pin

[Rogers.Gary]

After reading the data sheets, the Getting Started document and others, I need some clarification on connectivity of the VddUSB pin. On page 12 of AN4555, it states:

"The USB transceivers are supplied from a separate VDDUSB power supply pin. VDDUSB range is from 3.0 V to 3.6 V and its voltage level is completely independent from VDD or VDDA." I guess this means it's as dependent as the design makes it, or is it (also) isolated internally?

On my design, I have an STMPS2141 switch that is supplied by the same 3.3V that is feeding the MCU - the USB will be enabled only when a USB stick is plugged in. So does it matter if the VddUSB pin on the MCU is fed straight away from the supply, or should it too be supplied by the output of the STMPS2141?

Is anyone aware of a reference design with a USB on the STM32L4?

Thanks

[Tesla DeLorean]

Not stand-alone, but you could take a look at the EVAL, NUCLEO and DISCO boards.

[S.Ma]

Find the corresponding discovery and nucleo board.

Then check which one has USB OTG AND example code in Cube library.

If there is, check the schematic and try the code.

VddUSB decorellated from other supplies is probably coming for situation where MCU GPIO pins need working at 1.8V. Read the MCU spec supply voltage min/typical/max and notes to find out what is allowed from what's not.

[Rogers.Gary]

That makes sense. The fact that the MCU can run down that low would necessitate the need for a separated supply for proper handling of the USB signals. Since USB is 3V to 5V, that quantifies it. Page 7 of STM32L4 getting started states that "When the functions supplied by VDDA, VDDIO2 or VDDUSB are not used, these supplies should preferably be shorted to VDD."

It's now clear I can run the USB on the same supply (3V3) as the MCU and tie VddUSB to that same supply.

Thanks guys.

[Rogers.Gary]

It looks like what I said is not correct. Some reading reveals almost all flash drives require 5V. Does it mean if I supply the USB flash drive with a 5V supply, I still tie VddUSB to a range of 3.0 to 3.6V?

[waclawek.jan · Accepted solution]

VDDUSB supplies the transceiver pins (DP/DM) which signal at 3V.

The 5V the FLASH drive needs is its power supply.

JW

[Rogers.Gary]

Perfect, just the answer I was looking for.

[Khouloud GARSI]

Hi @Rogers.Gary​ ,

There is an application note dedicated for USB peripherals embedded in the STM32 MCUs.

I advise you to have a look on section "Power".

Link to AN4879:

https://www.st.com/content/ccc/resource/technical/document/application_note/group0/0b/10/63/76/87/7a/47/4b/DM00296349/files/DM00296349.pdf/jcr:content/translations/en.DM00296349.pdf

Khouloud.

[Rogers.Gary]

Very helpful, thanks.