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BQ76952: Constant offset in TS1 temperature using the internal temperature model

Manuj Agrawal1
Prodigy 175 points
Part Number: BQ76952

Dear TI Team,

I have been using a 47K NTC (NXRT15WB473) with the parameters as in the image below 

I have used the TI's thermistor coefficient calculator on the BQ76952 product page to calculate the 180K polynomial model parameters with 180K internal pullup.

  • Following this, I have observed constant positive offset in the temperature calculated internally on the TS1 pin by nearly 35 degree Celsius. 35 degree is beyond what I can correct using the "Calibration:Temperature:TS1 Temp Offset" settings.
  • I have tired replacing the 47K NTC with 10K NTC using 18K temperature model and internal 18K pullup, and the positive offset still exists. Parameters for 18K model for 47K NTC were beyond the range settings can store.
  • This positive offset is not observed with either 10K or 47K NTC on other pins like TS2/TS3/HDQ etc. The issue is only with TS1.  

Could you please help me with find a solution for this?

Thanks and Regards

Manuj 

  • 0
    TI__Guru* 81034 points

    Hi Manuj,

    This is very strange that the offset is only observed on TS1. All of the thermistor pins should behave the same. Is there anything different about your configuration settings for the other pins vs TS1?

    Can you share the polynomial you used?

    Regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 175 points

    Dear Matt

    I have filled and used the excel sheet for calculating the coefficients as in the image below

    coeff_ADC0 = 17246 for 180k model

    TS1_Config = 0x57

    TS1 offset = 0

    I have also configured CFETOFF and HDQ pins with same polynomial coefficients and pin config = 0x57 but haven't observed this big an offset.  

    Please let me know if you have observed similar issue before too. I have observed this issue with multiple bq76952 ICs now.

    Thanks and Regards

    Manuj 

  • 0 in reply to Manuj Agrawal1
    TI__Guru* 81034 points

    Hi Manuj,

    You mentioned you see the same issue with the 10k NTC? No other users have reported this type of issue. Can you export your complete register settings and send them to me?

    Thanks,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 175 points

    Hi Matt,

    I would like to connect a 10K NTC on TS1, enable internal 18K pull up and read actual voltage in mV across the 10K NTC from bq76952. I would later process this voltage across NTC (varying with temperature) along with this known value of internal pull up to deduce a more accurate temperature in our microcontroller.

    Can you suggest settings for "Settings:Configuration:TS1 Config" which I should use here and how should I read this voltage (instead of temperature from polynomial) from bq76952 (through any command).

    Regards 

    Manuj

  • 0 in reply to Manuj Agrawal1
    TI__Guru* 81034 points

    Hi Manuj,

    I am still puzzled why you are seeing different results on TS1 as compared to the other thermistor pins. They should behave identically.

    There is variation on the internal 18k pull up - the measured value of this resistor is stored during production and used as part of the temperature polynomial. So if you use the method you describe, I do not think you will get the best accuracy. If you want to use an external pull-up resistor of a very accurate value and use the ADC measurement of the pin, you can set TS1 Config = 0xB3 for the ADC measurement. Here is a document that describes some of the different settings options:

    BQ769x2_TemperatureRawCounts_Instructions.docx

    Best regards,

    Matt