NRF5340 High-Voltage and Low-Voltage mode on same circuit

[villanif]

I am currently designing a circuit with the NRF5340. This circuit will be alternatively supplied by VUSB when connected to a USB port and by a 1.8 V power supply to VDD when disconnected, which is, according to the datasheet, not possible when the circuit is configured in high voltage mode. 

From a forum post I saw that it was possible, altough not reccomended, to supply the voltage from VDD for short periods and leave VDDH floating. To allow the supply to run from VDD for longer periods, would it be possible to attach a MOhm-range resistor or a Diode between VDDH and VDD to pull VDDH to VDD and keep it at a stable voltage when the voltage is not supplied by the USB bus? Or are there some other alternatives? 

I have extremely tight space constraints, and have approximately 2mmx4mm space for this "High Voltage to Normal Voltage mode" circuits. 

Thanks!

[ketiljo]

The switch between normal mode and high voltage mode and vice versa required a power on reset. On the fly manipulations of the supply voltage like this is very unpredictable. So given that you have a separate 1.8 V regulator, it will be best to feed this with VBUS when USB is connected and battery (?) when not. 

[villanif]

Thanks for the answer! The reset is something easily settable in this case. The issue is that this circuit is intended to have two distinct operating scenarios: 

1.  Standalone, with a battery used to supply the internal NRF DC/DC converter that is then used to supply the 1.8 V power rail of the board.
2. connected to an "expansion board",  that supplies the 1.8 V to the board. 

The fallback option is to add a solder point that can be connected and disconnected depending on the usage. Alternatively, a switch could be connected to an output pin. The bottom line is: I do not have a separate 1.8 V regulator that I can always use, and I want to find out the smallest and easiest way to be able to use the board in these two operating modes (cutting power in between, as you have suggested).

I think what I need to know for this is what kind of resistance is acceptable between VDD and VDDH in normal mode and if it has to be at exactly the same voltage of VDD or if a small deviation would be allowed, as this would simply allow me to add a diode between VDD and VDDH, which would fix VDDH to a voltage near to VDD when the board is in "external supply mode" and block whenever the voltage of VDDH is higher (due to battery connection) in "standalone mode".

[ketiljo]

With a resistor between VDDH and VDD, you risk overvoltage on the VDD pin in system off where the current draw is low. So this is not recommended. 

The VDDH has been recommended to be connected to VDD. If VDDH is left open it will follow VDD by one diode drop below. It will probably work, so that low voltage mode is selected but no guarantee given. So the absolute best is to have a solder pad to short VDDH to VDD when the module is standalone.