Current Spike at boot (80mA)

[David K.]

Dear Amazing Community,

We are currently investigating any uA being consumed by our nrf52833.

We are using the PPK to track them and we believe that during normal operations, the nrf does its job well.

However at boot time, with the PPK and on a PCA10100 DK, we clearly spot a spike of 80mA of 55uSecs. 

Also, the same code but running on the 52840 allows for 25mA for approximately the same amount of time. Better, but still a lot for a boot.

Would there be any configuration we have missed to avoid this Spike and gain precious mA on the nrf 52833 (and 52840)?

Thanks for your everything being shared here,

Bests

David

[Jørgen Holmefjord]

Hi,

I believe what you are seeing is the inrush current for the decoupling capacitor of the VDD pins of the chip. This current may actually be higher than 80 mA, but the PPK will cap the current at 80 mA, as this is the max it can measure (spec says 70 mA).

There are approximate 6uF decoupling capacitors on the VDD/VDDH pins of the nRF52833 reference circuitry. Given 3V supply voltage, this will give a charge of 3V*6uF=18uC. If the spike you see is 55us long, this corresponds to 18uC/55us=327.27mA current draw for 55 us.

This will be the same for all designs that use decoupling capacitors (which almost all electronic designs do). If you use System OFF idle mode instead of cutting the power completely, you will not have to worry about this current after the power supply is enabled.

If you want to measure the boot-current of the device, you should put it in system OFF sleep mode and measure the current from exiting System OFF until the CPU is ready to run code.

There is no need to measure the inrush currents, they can be calculated from the supply voltage and capacitors values in your final design.

Best regards,
Jørgen

[David K.]

Hi Jørgen,

Thanks for answering. 

Since I am not the electronican of our team, I will let them read and react if necessary.

We are currently investigating batteryless systems, so the power might be cut at some point :-).

Bests,

David