Our nRF52832 application PCB's battery level is dropped every 1024 secs

[Daewoong Kim]

Hi,

I've developed nRF52832 chip for our company's application,

and I found some weird phenomenon

we using nRF52832 chip with Nordic softdeivce and SDK v12,

BLE chip operates with DCDC converter (internally),

The power is supplied by 3V coin cell.

The above figure shows VDD voltage of BLE chip. Those values measured by SA-ADC module in BLE chip

every 1 second, and our custom hardware keep stored in refrigeration room (about -20 C).

As you could see, the VDD value have dropped every certain period. The period is about 1024 secs.

Anyone knows that? why this phenomenon is being observed..?

Thank you for your helps

- Daewoong Kim

[Turbo J]

Daewoong Kim said:

power is supplied by 3V coin cell

Those have high internal resistance...

Daewoong Kim said:

hardware keep stored in refrigeration room (about -20 C).

That makes this even worse - resistance is even higher at low tempereature.

Dumb question: How do you store the SAADC values? That might just happen every 1024 seconds and  would need some more power than usual.

In any case you need some more bulk capacitance in your design.

[Daewoong Kim]

ADC values is sent to BLE central device and the central device prints that value using UART terminal.

I understood that the ambient temperature may have an negative effect on the BLE chip.

However, It just weird the ADC value of BLE VDD (Battery) dropped per certain period...

If the problem is caused by the capacitance of coin cell, why the ADC dropped phenomenon

is observed intermittently, not continuously..?

[Silvano]

Coin cells have voltage dips when driving quick bursts of current, for example, when doing a BLE transmissions. For example if your coin cell has a resistance of 10 ohms and your draw 15 mA of current for a BLE transmission you will have a 150mA drop in voltage at the instant from the battery. At least this is my understanding.

They way to avoid this as of today is to add a super capacitor after your battery and before your load. The downside is that 3V super caps in small packages are not yet widely available. Which means to use the super capacitors with common voltage rating is 2.7V or 2.5V you need to step down converted between your battery and super cap. This is not very efficient as you start to lose power on the leakage from your stepdown converter. You would have to the math on your own device parts to find out if it more efficient to do this or leave the battery by itself. 

This article explains it a bit better and shows what we can do when 3V super caps in size efficient packages become more available.

Potentially adding a decent bit of bulk capacitance as Turbo J suggested could help the problem with out adding a super capacitor into your design.

Hope this helps.

[haakonsh]

In addition to the previous answers, I'd like to add that your data has a bad case of aliasing, your sampling period is far too low. 
Also, due to the voltage drops previously explained, what you're really measuring is the power consumption of the device, but with a source that is unable to deliver the required load. I suggest you make a proper analysis of your power consumption by measuring the current consumption at a constant voltage and with a high sample rate. 

See Measuring current for more details.