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BQ78350-R1: Parameter description Rc0 and Adc0 from Cell Temperature Model sub-class is missing

Prasanna Thengodkar1
Prodigy 110 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350, BQ76940, , BQ76942

Hello, 

I am using TI excel tool to calculate the coefficients of the cell temperature model based on the NTC we are using. 

I am unable to find any explanation regarding parameters Rc0 and Adc0 from Cell Temperature Model sub-class in the technical reference manual.

Please let me know what these parameters are used for in cell temp. model and what these should be set to.

Thanks and regards,

Prasanna

  • 0
    TI__Guru* 81034 points

    Hi Prasanna,

    Here are the descriptions of these two parameters:

    Rc0: Temperature correction factor corresponding to ideal ADC measurement at the calibration temperature after scaling by 5/3. For example, using calibration temperature of 25C, assuming thermistor value of 10-kOhm, pull-up resistor of 18-kOhm, this is calculated as Rthermistor / (Rthermistor + Rpull-up) * 32767.

    Adc0: Temperature correction factor corresponding to actual ADC measurement at the calibration temperature, with a scale factor included. With the thermistor attached to TS1 at the calibration temperature, the TS1 Raw ADC Counts (in 32-bit format) is divided by 256, then multiplied by 5/3, and used for this setting.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 110 points

    Hi Matt,

    Thanks for the explanation. 

    Regarding Rc0, it looks like it is to account for the pull-up resistor value in the model and having correct pull-up value is important indeed then.

    We are using bq76940 along with bq78350-r1. The pull-up is internal to bq76940 and datasheet page 17 says its 10k ohm. Using 10k ohm gives me the wrong coefficient from the excel tool. Is this a mistake in datasheet or mistake in the tool? If I use 18k pull-up value, the calculation gives me Rc0 = 11702.5 which is also the default value in bq78350. Is this correct value to use if we use 10k NTC?

    Adc0 is still not completely clear to me. Is this to account for tolerance in pull-up resistor? This parameter looks like the output of the temperature calibration process, what should it be set to default? 

    Look forward to hear back.

    Prasanna

  • 0 in reply to Prasanna Thengodkar1
    TI__Guru* 81034 points

    Hi Prasanna,

    The descriptions I sent came from a different battery monitor device BQ76942, so the 18k pull-up does not apply to the BQ76940. The BQ78350-R1 document does not describe these parameters, but the recommendation is not to modify Rc0 or Adc0 from the default values. I have been told the BQ78350 Thermistor Coefficient Calculator already accounts for this difference in the calculations.

    https://e2e.ti.com/support/power-management/f/196/p/637657/2352741

    https://e2e.ti.com/support/power-management/f/196/t/541826

    Best regards,

    Matt