VBUS supply before system ON on normal voltage mode nRF52833

[deep_dive]

1. Our system is configured with circuit config3 of the nRF52833 WLCSP, where an external power supply of 1.8V is applied to VDD.

Power sequence : USB connection -> VBUS supply -> external LDO 1.8V output -> nRF52833 VDD supply 

Due to the turn-on time delay of the external LDO, the V_USB pin (for VBUS & USB Type-C detection) is supplied before the VDD 1.8V of the nRF52833.

Is it OKAY?

In other words, is it okay for V_USB at 5V to be supplied before VDD at 1.8V in a system configured for normal voltage at 1.8V?

2. According to the datasheet section 5.3.2,when the system is OFF and VUSB is applied, the system turns ON.

Does this mean that the system will turn ON regardless of whether the system circuit configuration is in high voltage mode or normal voltage mode?

3. If the system turns ON when VBUS is connected while in normal voltage mode and the system is off, I thought this is because the VUSB LDO is working.

Are the VUSB internal LDO and the VDD internal LDO different from each other?

[Simonr]

Hi

So, I just got a confirmation from my colleagues on this.

1. It's okay to supply VBUS before VDD. The system will not attempt to start up until VDD is valid. However, this is a bit in the grey zone from an analog design point of view, but it should be okay here.

2. No new input.

3. The LDO from the VBUS to USB is indeed dedicated to the VBUS net, and is different from the internal regulator LDO.

Best regards,

Simon

[deep_dive]

In our system, which uses config 3 that supplies external 1.8V to VDD, I understand that even if VBUS is applied before VDD, the system will not turn on until VDD becomes valid.

Simonr said:

1. It's okay to supply VBUS before VDD. The system will not attempt to start up until VDD is valid. However, this is a bit in the grey zone from an analog design point of view, but it should be okay here.

So, I'm a bit confused about the answers to questions 1 and 2.

Regarding question 2, in our system, even if VBUS reaches a valid range, if VDD is not supplied, the system won't run the startup, right? Please clarify this part.

Simonr said:

2. When the device is in system OFF, and VBUS trigs a reset because it reaches a valid range, the device resets and thus will run the startup as when you first start up the device, in whichever is set by default (VDD/VDDH).

I have an additional question.

4. In our system, since the system turns on by VDD after VBUS is applied, does the USBDETECTED event occur right after VDD is turned on?

5. And is there a defined allowable time between VBUS being applied and VDD being supplied for the system to turn on?