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BQ34110: External Thermistor Coefficients

Simon Marker
Prodigy 105 points
Part Number: BQ34110

Hi,

I am trying to determine the coefficients to use with a non 103AT thermistor.

I have found the Excel tool in slua398, but I'm having difficulty getting it to work even if I put in the default 103AT parameters.

One confusing thing is that the thermistor on the bq34110 pulls the TS pin up to the 2.5V reference. The technical reference manual then describes a pull down being implemented internally and the data sheet says that that TS pin input impedance is 5k when the pull down is on. 

The sketch given in Thermistor Coefficient Calculator.xls from slua398 shows the TS pin with a fixed pull up and the thermistor pulls this to ground.

How do I set the parameters in the calculator to account for this difference? If I put the 103AT resistance curve in I would expect the parameters generated to match the default ones, but I can't get them anywhere near at the moment.

Thanks,

Simon

  • 0
    TI__Mastermind 19195 points

    Hi Simon,

    Any NTC close to  Semitec 103AT or Mitsubishi BN35-3H103FB-50 will work. This is 10.0k with beta =  3435 Ｋ

    Best regards,

  • 0 in reply to Nick Brylski
    Prodigy 105 points

    Hi Nick,

    I'm not looking for an alternative to a 103AT.

    Given that the device supports custom thermistor configurations I just when to be able to set the Ext Coeff parameters in the dataflash to values that match the thermistor that I'm using.

    If there isn't a tool available to do this, can I send you the table of T/R values for you to generate these please?

    Thanks,

    Simon

  • 0 in reply to Simon Marker
    TI__Mastermind 19195 points

    Hi Simon,

    Please attach the table here and I'll take a look.

    Best regards,

  • 0 in reply to Nick Brylski
    Prodigy 105 points

    Hi Nick,

    Here is the data:

    -20 124222.2222
    -15 91788.88889
    -10 68466.66667
    -5 51544.44444
    0 39133.33333
    5 29966.66667
    10 23122.22222
    15 17977.77778
    20 14088.88889
    25 11111.11111
    30 8824.444444
    35 7053.333333
    40 5673.333333
    45 4590
    50 3735.555556
    55 3056.666667
    60 2514.444444
    65 2078.888889
    70 1727.777778
    75 1442.222222
    80 1210

    Thanks,

    Simon

  • 0 in reply to Simon Marker
    TI__Guru** 105760 points

    Hello Simon,

    We really recommend using the 103AT, lots of times using a different thermistor results in many issues as the models never match well.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    Prodigy 105 points

    Hi Wyatt,

    Thanks for the reply, but this is not an option for me.

    If TI don't want to support this feature, then why expose the parameters?

    I quite accept that there is no guarantee that the algorithm will be able to match the same accuracy as the 103AT, but that is part of the overall system design trade-off choices. The approach that is used on the calculator for the 78350 is a good one as it graphs the accuracy and gives the option to move the optimisation point. 

    I assume from the responses from TI so far that there isn't a calculator available for this device.

    If you are also unable to provide me with a set of parameters for the dataset that I have posted then I'll have to address the issue by accounting for the error when I program temperature parameters and translating the reported temperatures, both of which are far from ideal.

    So please answer yes or no, are TI going to support this feature?

    Thanks,

    Simon  

  • 0 in reply to Simon Marker
    TI__Mastermind 24870 points

    Hello Simon,

    It will be difficult for us to guarantee any type of operation.

    Please look at this application note to see if it will help.

  • 0 in reply to Kang Kang
    Prodigy 105 points

    Hi Kang,

    This doesn't really move the discussion on.
    This app note just references out to slua398 as a coefficient calculator which is not applicable to this gauge as the physical thermistor divider is inverted compared to the assumption in slua398 as per my original post.

    The core question still remains, how can the coefficients be calculated for this gauge?

    Simon

  • 0 in reply to Simon Marker
    TI__Mastermind 19195 points

    Hi Simon,

    How far off are the default coefficients from the thermistor you are referring too? 

    Best regards,