nRF52840 questions about power consumption in sleep mode

[Maxime]

Hi There,

I'm a beginner with Nordic products and I'm taking over the nRF52840 Preview Development Kit.

My final goal is to supply with a solar cell my nRF52840 running in Low-Power mode and sending advertising at regular intervals.

--

So, I would like to measure the current of the chip in sleep mode with an ampere meter. To achieve this, I have followed this document :

http://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52%2Fdita%2Fnrf52%2Fdevelopment%2Fnrf52840_pdk%2Fhw_measure_current.html

- I used external power supply (3V) on connector P21.

- I cutted the PCB track shorting solder bridge SB40.

- I connected an ampere-meter between the pins of connector P22.

- Switch SW6 is on “nRF only”.

- Switch SW9 is on position “VDD.

NOTE : maximum resolution of the ampere-meter : 10 nA

--

To measure, I used the following example program :

<InstallFolder>\examples\ble_peripheral\ble_app_beacon

- I have commented the advertising_start() function (in the main function) to stay in sleep mode.

- I have commented the BSP_init() function to reduce consumption.

NOTE : I used nRF5 SDK v12.3.0 and S132 softdevice.

--

My problems are as follows:

- The current given by my ampere-meter is 500uA, which is far too high in standby mode according to specifications. How can I reduce that ?

- By measuring with the ampere meter directly after the external power supply, I have about 2.5 mA which is even more and which does not correspond to my previous measurement. What does the current measured on P22 correspond to ? Later, where should I connect my solar cell ?

--

Can you please help me understand and solve my problems?

Thank you for your answers !

[Bjørn Kvaale]

Hi,

Although you can use the nrf5 sdk v12.3.0 & S132 softdevice (see link), it is not recommended. "The SoftDevice S132 v3.0.0 has limitations when running on nRF52840 and should only be used in the context of demonstration and experimenting with the new device."

Could you try the same setup using the latest sdk 14.2 & using the latest S140 softdevice?

As this link says, the P22 current measurement measures the current in series with the nrf5 device.

You could try connecting the solar cell to the P21 external supply pin like described here, but I am unsure whether your solar cell will be able to provide enough voltage & current for the 52840 chip. You could also try switching the switch on the top right of the PDK to nRF Only, which will cut off power from the interface MCU & the LEDs. This could be useful for your low power beacon application.

I think it could be useful for you to check out the solar cell reference design Nordic has developed. The firmware & hardware descriptions are given there.

[ElectroMax]

Thank you for the answer!

It's much better with the last sdk 14.2. & using the latest S140 softdevice. (the current consumption in sleep mode is about 2-3 uA)

I used the P22 connector for my measurements, everything worked!

I tried connecting the solar cell to the P21 external supply pin like discribed in the document "nRF52840 direct supply" .

The problem is that the current coming out of my solar cell is much greater than when I measured on P22. I have noticed that when I use an external power supply instead of my solar cell, the current decreases when I lower the voltage. Perhaps the additional consumption is due to the DCDC converter of the SoC ?

[Bjørn Kvaale]

How much are you lowering the voltage by on your external power supply & how much is the current decreasing by? Also, what are start & end voltages & currents of your measurements?

Once extensive testing has been done on the nrf52840 chip, you can take a look at this link for more info regarding power consumption (see this link too).

[ElectroMax]

Hi Bjorn,

thanks for your help !

I was able to have the desired consumption with this link: 

http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52/dita/nrf52/development/nrf52840_pdk/direct_supply.html

it says :

Since it is only the nRF52840 SoC that is supplied from this source, it is recommended to supply the VDD domain from a different source (coin cell battery or USB) to prevent the pins of the SoC to be connected to unpowered devices. To prevent leakage due to voltage differences, the board should be set in the nRF only mode, see section Operating modes.

I had the switches in the following positions : 

- sw10 : On
- sw6 : nrf Only
- sw9 : VDD

I used USB source J2, as it says in the link.

Without the USB source, I'm consuming 90uA in sleep mode. With I'm consuming 2uA.

So i'm thinking that if I do not want to deal with the USB source, I need to create my own PCB with only a nRF and your power supply. Is it correct ?

[Bjørn Kvaale]

Hi Maxime, it does seem like this could be the case. You could either create your own PCB, but it might be a good idea to take a look at these third party beacons here. Like mentioned previously, it might be an idea to take a look at the nrf51 solar cell reference design. In addition, this reference circuitry could be useful.