VDDUSB Connection on STM32F7

[kevin2399]

Posted on November 02, 2015 at 16:12

I have a very size- and power-constrained application for the STM32F7 series.  It will not make use of USB peripherals.  Is it possible to tie VDDUSB to GND and eliminate the recommended bypass caps to save space, or do I need to supply it with either VDD or an external supply and bypass it?  The block diagram makes it look independent from the other systems. The reference manual only states that one of the supplies is required for the USB transceivers.

[re.wolff9]

Posted on November 02, 2015 at 16:24

First: Check the datasheet electrical specifications. If it says something about VDD and VDDUSB needing to be within XXX from each other the below probably applies....

Usually the chip manufacturing process creates a whole bunch of ''parasitic'' components that can usually be ignored, provided you abide by certain rules. Often these parasitic components are diodes that under normal circumstances are reverse biased and won't conduct any current. But when you do something like not power the VDDUSB or even tie it to ground, you are probably violating the ''normal'' rules and those diodes start conducting. 

That said, the bypass capacitors are there to provide current when the module outputs switch levels. If your design guarantees that no switching will occur the capacitors are not necessary. I expect things to work if you connect the pin to VDD. 

update: I've checked the datasheet for you. Under absolute maximum ratings the VDDUSB is required to be within 50mV of all the other VDDs. Tie it to ground and you aren't allowed to power your chip. :) 

[kevin2399]

Posted on November 03, 2015 at 23:13

Hi Roger,

Thanks for that reference in the Absolute Max section. I interpret that differently, i.e. I think it refers to the pins that are identified as VDD (of which there are many). I think they are saying the voltages on the VDD pins cannot vary by more than 50mV.  I also read elsewhere that VDD and VDDA can vary from each other by as much as 300mV during power up and power down, which is greater than the 50mV spec. However, you did set me on the right track - I found a spec for VDDUSB further down, which states that if USB is not used, the supply voltage must be between 1.7 and 3.6 V, so I guess I will tie it to VDD, which has the same spec.  Thanks for your help.