nRF54L15 high power consumption when VDD is less than ~2.29V

[cyber_v1]

Hello,

We have a custom nRF54L15 board, that, when powered with 1.8V, shows a too high power consumption. 230uA in system on - low power mode, and 12uA in system off mode.

When it's powered with 2.29V or higher, it shows much better numbers of 6uA and 1uA, respectively. The 2.29V is borderline - anything slightly below, and sometimes it starts up with the higher power consumption, sometimes it starts up ok. At around 2.25V maybe, it's quite consistently starts up with the high consumption.

Changing the voltage without the powering the device off and back on again doesn't affect the power consumption. We thought that maybe DC/DC regulator is not getting enabled, but it does (checked the registers, and waveform at DECD with an oscilloscope).

The board is based on the QFAA reference design, and all custom parts have been disconnected while investigating the issue.

Any ideas what may be wrong, or what to check to investigate this issue further?

[Kazi Afroza Sultana]

Hello,

 The load frequency of the crystal oscillator you are using is 9pF. Its in the require ranged according to product specification. 

nRF54L15 supports internal capacitors for the LFXO. If the chip has internal capacitors, following formula is used for calculating INTCAP register.

INTCAP = round( (2*CAPACITANCE - 12) * (FICR->XOSC32KTRIM.SLOPE + 0.765625 * 512)/512 + FICR->XOSC32KTRIM.OFFSET/64 )

Can you try to check that INTCAP register is correctly calculated for your crystal's 9 pF load capacitance?

[cyber_v1]

Kazi,

We verified it and it seems CAPACITANCE in the formula is the capacitance the crystal must see. After changing it from 17000 to 9000 it works! Around 8uA with barebones k_sleep(FOREVER)!

I'd say the documentation is quite confusing; it shows how CL is calculated, and then goes on about the configuration of the internal capacitors, and under the formula: "CAPACITANCE is the desired capacitor value in pF". So one might think the CAPACITANCE is Cᴵᴺᵀ, but it's CL? Could you please confirm?

(The documentation of the load-capacitance-femtofarad in nordic,nrf-lfxo could also be improved)

Thank you for your help!

[Kazi Afroza Sultana]

Hello,

I agree with you. We should explain this in the datasheet not to confuse the reader.

For now, CAPACITANCE value is the load capacitance CL from the crystal datasheet. 

I have talked to team about this. Hopefully there will be an updated version of this chapter on upcoming version.

Thanks.

BR

Kazi