Power Sequence of NRF52840

[Hani.Abidi]

Hello,

I have a question regarding power - up sequence of NRF52840. 

When the USB is plugged in, the VBUS voltage is at 5V.

However, at the same instant, VDDH is not powered. 

When the user closes's SW1 switch, LDO is enabled and VDDH is powered at 3.6V. 

Their might be a delay of minutes between VDDH getting powered and VDDH getting powered. It's my assumption that VDDH and VUSB have different power blocks internally. Is this scheme a violation of power-up sequence?

Thanks,

Hani Abidi

[Simonr]

Hi Hani

 You're correct that VDDH and VBUS have different internal voltage regulators. You can see more details on this in the Main supply and USB supply chapters in the product specification.

a delay of minutes between VDDH getting powered and VDDH getting powered

 I'm sorry, but what do you mean here?

The setup you describe is not okay, as VBUS and VDDH must be powered together, as both VBUS and either VDDH or VDD supplies are required for USB peripheral operation. When VBUS rises into a valid range, the software is notified through a USBDETECTED event (which is part of the USB peripheral) which in turn requires VDDH to be powered as well as VBUS. Keep in mind that the LDO should not be necessary though, as the VDDH pin is able to handle a 5V input as well. 

Best regards,

Simon