NRF52840 USB VCC requirements

[lo]

Hello,

We are using NRF52840 and for a specific use case we will install our module on an M.2 (A/E) breakout. In this configuration USB will be the main communication interface and this lead to an important question : can we use the USB from the NRF52840 with only 3.3V VCC?

Bringing 5.0V to this breakout would required to add a dedicated DC boost which we really want to avoid (for various reasons) so we wonder wether there is a way to use the USB interface of the NRF52840 when providing only 3.3V VCC, even if some trick is required (for example powering the USB core with 3.3V directly from the USB_DEC pin).

The matter is rather urgent , any answer will be very welcome.

Thank you in advance.

[Turbo J]

Nope. V_Bus minimum is 4.35V, see Power chapter in the PS. Lower voltage will most likely not power up the PHY properly (USBD power up sequence in the PS requires valid V_Bus).

[lo]

Thanks, i have seen that requirement, but i have also seen that VBUS directly goes to an LDO and the PHY itself is actually powered by 3.3V:

The USB peripheral has a dedicated internal voltage regulator for converting the VBUS supply to 3.3 V used by the USB signalling interface (D+ and D- lines, and pull-up on D+). The rest of the USB peripheral (USBD) is supplied through the main supply like any other on-chip feature.

That is what lead me to think it may be possible to power the USB PHY directly with 3.3V from DECUSB which is just after this LDO, the only problem would be the VBUS detection, which according to datasheet seems to be done before the LDO with a minimum of 3.4V.

VBUS,DETECT Voltage at which rising VBUS gets reported by USBDETECTED 3.4 - 4.0 - 4.3 V

[Turbo J]

An USBDETECTED voltage of about 4V won't help much you if there is only 3.3V available.

Remember statistics? I would expect a gaussian distribution - many chips with around 4V USB detect threshold, and very very few at the extremes (3.4 and 4.3V).

[lo]

Yes i totally agree that, even if 3.4V was enought it would likely never be reached anyway. So i think our only option is to add a DC boost on the M.2 breakout.