A 5 V USB supply must power the VBUS pin when using the USB peripheral.
The device has a dedicated internal voltage regulator for converting the VBUS supply to 3.3 V for the USB signaling interface (D+ and D- lines, and pull-up on D+). The rest of the USB peripheral (USBHS) is supplied through the main supply like other on-chip features. As a consequence, both VBUS and VDD are required for USB peripheral operation. For details on the device voltage regulators, see REGULATORS — Regulator control.