nRF52840 High Voltage Mode Specs

[anse008]

Hi, I've been measuring a nRF52840-powered prototype powered from a 3V coin cell battery. I would like to use High Voltage Mode with REG0 in DC/DC-mode for maximum battery life.
However, I see Battery voltage drooping very low due to changes in MCU load, BLE transmits etc. I have couple questions:
1: What happens if Vddh momentarily drops below specified minimum of 2.5V? Overall, the device seems to work but..?
2: If Vdd is set for example to 1.8V, may Vddh drop to 2.1V then? REG0 should still regulate, but are there other consequences?
3: What if one would increase Caps on REG0 output (VDD) well over specified max 5.5uF? (in effort to lessen the effect of load transients in VDD)

[ketiljo]

When you have a battery voltage below 3.6 V, it's better to use the device in normal voltage mode with REG0 turned off. The device will operate down to 1.8 V and the battery is empty well before then. There's no benefit of using REG0 unless the supply voltage is above 3.6 V

1. VDDH needs to be > 0.3 V above VDD. Else REG0 will not be able to regulate properly. 

2. It should work

3. The regulator may be unstable if you exceed the max capacitance on the VDD side. 

[anse008]

Thanks!

Are You saying that REG0 efficiency below 3V6 is such that one should never go there? We do have some other digital circuits connected to VDD. Hence my guesstimate has been that using REG0 would pay off (especially when VDD is 1V8). 

1&2: Unless question above dictates otherwise, I think we'll proceed in High Voltage Mode with VDD set to 1V8 or 2V1. This should allow us to drain coin cell quite empty.  With an emptyish battery, BLE tx transients do cause VDDH to dip well below 2.5V but MCU seems to work fine..

3: Roger. We'll use 4u7 caps on REG0 output.

[ketiljo]

Hi

Not necessarily. But as the regulator will not be 100 % efficient, you will have more loss when running the device of REG0 instead of directly from the battery. The regulator needs to have a 0.3 V  voltage drop while without you can operate all the way down to 1.8 V. The normal voltage mode will be able to drain the drain the battery just fine. The battery is close to empty when the voltage is 2.4 V, unloaded. So unless you require a stable supply voltage for other circuitry, just run the device in normal voltage mode. 

cheers,

Ketil

[anse008]

Thanks again for this info.

Our measurements do show that even with our external loads Normal Voltage Mode should do nicely. Seems that even longer battery life can be achieved in NVM, depending "a bit" of FW. Some more testing will tell..

Case Closed for me..