• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 343 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ25887: Question regarding the calculation of RT1 and RT2 for the thermistor in the data sheet

Christian Staudigl
Prodigy 10 points
Part Number: BQ25887

Tool/software:

Hey,

I have a question about the calculation of the resistors for the voltage divider of the thermistor on page 23 of the data sheet. The values used for V_T1 to V_T5 seem inconclusive to me.
In the table for the electrical properties of the chip on page 9, these are given as a percentage of the REGN voltage. When I calculate these, however, I arrive at completely different values than those that appear to have been used in the data sheet in the calculation on page 23. If I use the values from the table, I get negative / incorrect values for the resistors RT1 and RT2. I have attached the corresponding calculations at the end of the post.

My questions are the following:
1. how do you arrive at the values for V_T1 and V_T5 from the calculation on page 23?
2. if I have interpreted the values for V_T1 and V_T5 from the table correctly, why do I get negative values for RT1 and RT2?

If I have made a mistake in the calculations or misinterpreted something in the data sheet, I would also be pleased to receive clarification in this regard :)

Best regards, Christian.

5873.Calculations.pdf

  • 0
    TI__Guru 57341 points

    Hi  Christian,

    1. Please refer to JEITA THERMISTOR COMPARATOR (BOOST MODE) on page 9 of the d/s for VT1 and VT5.

    2. For RT1 and RT2 calculation tool, please refer to http://www.ti.com/lit/zip/slurax7 for the BQ2589x calculator tool. Although it was developed for the BQ2589x, the principle is similar and can be used for BQ2588x with minor changes.

    3. The d/s uses 103AT NTC as an example. 

    4.  The RT1 and RT2 equations are valid for other types of thermistors.

    5. If RT1 or RT2 is a negative value, then the thermistor does not work and please try another thermistor. 

    Thanks,

    Ning.

  • 0 in reply to Ning T
    Prodigy 10 points

    Hello Ning,
    thank you for your detailed reply. I have checked the individual points again, but unfortunately this has not led to the solution to my problem.  Since I did not specify it directly in the post, the data sheet I am using is the version: SLUSD89B - FEBRUARY 2019 -REVISED NOVEMBER 2019 of the BQ25887


    To the corresponding points:
    1. in my calculation where I arrive at the negative values, I have taken the values VT1 and VT2 from the data sheet on page 9. For V_REGN I have assumed a value of 4.8 volts as specified in the data sheet.

    2. The formulas used in the Excel file differ greatly from the formulas in the data sheet of the BQ25887. I have compared the two formulae in the attached image. In particular, swapping the minuend and subtrahend causes the sign to change during the calculation. In addition, the result is scaled with V_REF in the Excel sheet. Judging by the fact that a positive value comes out there, the formula in the data sheet seems to be wrong?

    3. I also use the 103 AT NTC. Or to be more precise, I only use the values from the data sheet of the BQ25887 directly for the calculations without any external reference.

    4. Many thanks for the information. This will be important for the further implementation process!

    5. Thanks for the explanation, this is also important for further implementation. However, since I only use values for the thermistor used in the data sheet of the BQ25887, as described in 3, or in general values directly from the data sheet, this should not apply here.

    I hope I have been able to clarify exactly where my problem lies.
    Thanks again for your help!

    EDIT:
    The scaling with V_REF in the formula for RT2 from the Excel sheet seems to be completely truncated, which in my opinion means that V_REF has no effect on the result for RT2. Nevertheless, the main difference remains the swapping of the components of the differences. For RT1, on the other hand, V_REF remains relevant according to the formula and thus differs from the formula for RT1 from the data sheet.


  • 0 in reply to Christian Staudigl
    TI__Guru 57341 points

    Hi,

    VREF gets cancelled out in the equations as you mentioned. So VREGN does not impact on RT1 and RT2 values.

    Thanks,

    Ning.