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BQ76952: Temperature accuracy without calibration

Jessica Riley2
Prodigy 220 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQ76942

Hello,

I am trying to figure out how accurate the BQ76952 temperature sensor inputs are without performing calibration.

I am using a 10k NTC (TDK NTCG163JX103DT1S, ±0.5% resistance, ±0.7% B-value).  I have configured the BQ inputs to use the internal 18k pull-up resistors with NTC coefficients defined using TI's bq769x2 Family Thermistor Coef Calculator Tool (attached).  

Without doing any calibration, I am seeing a pretty high temperature measurement error (~90C where my reference sensor is 68C).  I was expecting it to be much better than this based on the datasheet information that the internal pull-up resistors are factory-measured.

What is the maximum expected error I should see using the uncalibrated temperature sensor inputs?  

bq769x2 Family Thermistor Coef Calculator - NTCG163JX103DT1S.xlsx

  • +1
    TI__Guru* 81034 points

    Hi Jessica,

    The online tool is the best public tool we have available, but my colleague developed an internal software tool that is able to generate the coefficients for better accuracy. We hope to release this as an online tool in the future. I calculate with these new coefficients below, the max error should be around 0.8C.

    A1 = -10923

    A2 = 5461

    A3 = 5461

    A4 = 21845

    A5 = 5461

    B1 = -19233

    B2 = 17147

    B3 = -7257

    B4 = 4409

    ADC0 = 11703

    Thermistor measurement accuracy is dependent on multiple variables including the accuracy/variation of the thermistor itself, so it is not easy to specify an accuracy based only on the IC.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 220 points

    Thank you for the quick reply and the updated coefficients @Matt!

    I will try repeating my test with these new numbers.

    Also, can you tell me what the maximum measurement error can be expected for the internal AFE temperature measurement, assuming no calibration?

  • 0 in reply to Jessica Riley2
    TI__Guru* 81034 points

    Hi Jessica,

    The max error is not specified for this parameter. There is only a typical spec on delta-Vbe. 

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 220 points

    Thanks Matt.

    Regarding the internal temperature sensor: I see from the BQ76952, BQ76942 Calibration and OTP Programming Guide that an external temperature sensor may be used to calibrate the internal temperature.  However, I am concerned that, given the IC is self-heating while it is on (we are using the built-in REG1 and REG2 regulator outputs), the temperature inside the IC will be warmer than any measurement made external to the IC through the plastic package.

    Can you tell me whether it would be better to leave the internal temperature measurement uncalibrated (with unknown max error) or to calibrate it with an external sensor which will result in an adjusted temperature some amount lower than the actual temperature internal to the IC?

  • 0 in reply to Jessica Riley2
    TI__Guru* 81034 points

    Hi Jessica,

    I don't think most users calibrate this parameter. You would need to control the ambient temperature accurately, let the board stay at this temperature to reach thermal equilibrium, and calculate the temperature of the die based on the IC RθJA in the datasheet. In the application, the IC will always be at a different temperature than the sensor readings - the battery cells with change temperature based on charge/discharge current while the IC should not but may be heated indirectly as the pack gets hotter. 

    It may be possible to do some rough calibration if the measurement is off by a large amount.

    Best regards,

    Matt