In AN70707.pdf（FX3 Hardware design guidelines...）,  "VBATT  is  a  power  input  to  the  USB  PHY.  It  is 

internally  regulated  to  3.3  V.  When  VBUS  is greater  than  3.7  V,  VBUS  becomes  the  primary supply  to  the  USB

circuitry  unless  there  is  a  software  override.  VBATT  pin  can  be  left  open  if  not used."

I'm confused, please explain and provide an example.

Hi,

VBATT is needed if you are using FX3 as a USB host. That means you need to connect a battery circuit to supply VBATT.

If you are using FX3 as a USB device then VBUS should be good enough to power the USB I/Os.

Even if you connect both VBUS and VBATT, FX3 will consider VBUS as the primary supply as soon as it becomes more than 3.7V.

Please let me know if this is not clear.

Thanks,

sai krishna.

I'm clear. Thank you!