BQ27220: External NTC reporting different temparature depending on current

Mark Egloff

Part Number: BQ27220

I'm currently testing the accuracy of the external NTC temparature reading of the BQ27220 and found that the temparature differs by almost 2 degrees Celcius depending whether the battery provides 10mA or 1800mA.

Setup:

External temparature 40 C, LiFePO, 95% charged

(1) I_BATT = 1800 mA, V_BATT = 3.1V --> Temp_NTC = 28.9 C (already converted from 0.1K)

(2) I_BATT = 9mA, V_BATT = 3.25V --> Temp_NTC = 37.1 C

The internal NTC reading also behaves similarly, but not as much as the external one.

Thanks for feedback on this behavior,

Mark

over 7 years ago

0 Onyx Ahiakwo over 7 years ago

TI__Guru* 76230 points

Are these tests being run on an evm or are they being run on your board? Are you seeing similar behavior on the EVM even after calibration?

0 Mark Egloff over 7 years ago in reply to Onyx Ahiakwo

Intellectual 380 points

Hi Onyx

It's on our own board. I'll take a look at it with the EVM.

Have a nice weekend,

Mark

0 Mark Egloff over 7 years ago in reply to Onyx Ahiakwo

Intellectual 380 points

Hi Onix

So, I've tested the issue with a GDK and BQ27220EVM and saw the same issues (even after calibration). I think the difference in temparature is related to the voltage drop of the battery when a load is attached.

Here's a graph of the logged data for the internal and external temparature depending on the voltage (please note that the change happened in a matter of seconds, so it's not related to the actual warming up of the NTC/IC):

I think 2-3 C difference will not have much of an impact on the gauging accuracy, right?

Thanks,

Mark

0 Onyx Ahiakwo over 7 years ago in reply to Mark Egloff

TI__Guru* 76230 points

yes that is right. 2-3 C will not have that much of an impact in accuracy.
thanks
Onyx

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BQ27220: External NTC reporting different temparature depending on current