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BQ24109: bq24109 RT1 and RT2 Calculations

Larry Norman
Intellectual 855 points
Part Number: BQ24109

The datasheet SLUS606P page 27 gives an example of calculating the TS resistors RT1 and RT2.

In the example, using RTHcold = 27.28kΩ and RTHhot = 4.912kΩ is supposed to yield RT1 = 9.31kΩ and RT2 = 442kΩ.

When I do the calculations given using these values, I get RT2 = 442.24kΩ, however, RT1 = 9.264kΩ.

Am I wrong or is the example wrong for RT1?

Thanks,

Larry

  • 0
    TI__Mastermind 34365 points

    Dear Larry,

    These values are nearly identical. The resistors chosen were used to match the ideal value to the closest realized resistor value. You can back calculate the effect of using these resistors and see that the temperature range of 0C to 45C corresponds to the ideal TS range of 34.4% to 73.5% of VTSB. However, with the realized resistors and thermistor in the datasheet the new TS range is 34.3% to 73.4% of VTSB, meaning the designed temperature range is ever so slightly off of the 0C to 45C range.

    Thanks,

    Mike Emanuel

    Please click "This resolved my issue" button if this post answers your question.

  • 0 in reply to Michael Emanuel35
    Intellectual 855 points

    Hello Michael,

    Thanks for the response, and I do understand that thermistor monitoring is not a highly precise activity.

    I did the reverse calculations to determine the actual voltage drop at TS; my calculations are correct, so unfortunately this is indeed a typo in SLUS606P.

    In many documents, Texas Instruments supplies a "Typical Application"  which is incredibly useful. I always use the calculation results from the example to ensure that I have my equations correct. It is a safe assumption that others do the same, and it can be confusing when examples in documents are incorrect, especially with the somewhat complicated equation used to achieve the value.

    I think I see what happened here. In SLUS606P, the "Typical Application" inductor on page 25 has a "calculated" (7.06µH) and a "selected" (10µH) value. For the TS divider on page 27, apparently the "selected" RT1 value of 9.31kΩ (common) was inserted in the text, and the actual "calculated" value of 9.26kΩ (very rare) was not referenced.

    Thanks,

    Larry

  • 0 in reply to Larry Norman
    TI__Mastermind 34365 points

    Dear Larry,

    I will add a recommendation for a datasheet update in the future. Keep in mind that any exact value for a component may not be a realized value that can be purchased. As such, the user has to make a choice to see if it is better for the system design to make the component value accurate yet more costly, or close enough and less costly.

    Thanks,

    Mike Emanuel

    Please click "This resolved my issue" button if this post answers your question.

  • 0 in reply to Michael Emanuel35
    Intellectual 855 points

    Hello Mike,

    Thanks for the response, and I do understand that the exact calculated value is not always the most practical (hence my discussion above of "calculated" vs "selected").

    I actually created a spreadsheet to calculate all of the equations for the bq241xx support circuitry, with additional cells to enter selected values to see the actual results of those selections.

    It will be good to get the datasheet updated to contain the corrected math.

    Thanks,

    Larry