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Webench/BQ25890: Discrepancy/problem when calculating RT1 and RT2 using Webench and using Equation 2 from BQ25890 datasheet.

Minh Nguyen
Prodigy 20 points

Part Number: BQ25890

Tool/software: Webench

Hello,

I found the discrepancy result of calculating RT1 and RT2 for TS resistor network using Webench and Equation 2 from BQ25890 datasheet.

When using the Webench, it does not mater what thermistor being selected, the vaule of RT1 and RT2 are always fixed with RT1 = 5.23KOhm and RT2 = 30.1KOhm.

For example, two muRata thersmistors NCP18XV103J03RB and NCP18XW153J03RB are selected, Webench report the same value for RT1 and RT2 in both cases. However, when I calculate RT1 and RT2 using Equation 2 from BQ25890 datasheet with 0°C to 60°C temparature range the result are as the following

In the case of NCP18XV103J03RB : RT1 =4.22KOhm and RT2 = 17.99KOhm

In the case of NCP18XW153J03RB : RT1 =7.85KOhm and RT2 = 45.39KOhm

Attached is the excel file "TI BQ25890 Thermistor Qualification Calulation.xlsx" I wrote to calculate RT1 and RT2. In the excel file, I included all formulas and  snippet of data sheets. The value highlighted in yellow are used for calculation RT1 and RT2.

Questions:

1.) Why does Webench always report the same value of RT1 and RT2 for difference thermistors?

2.) What temperature range does Webench  use to calculate RT1 and RT2?

3.) Please review my excel calculation and explain the discrepancy result between Webench result and Equation 2 result. Which result are correct?

Thank you,

Minh NguyenTI BQ25890 Thermistor Qualification Calculation.xlsx

  • 0
    TI__Mastermind 29010 points

    Hi Minh,

    Thanks for the question regarding thermistor selection. The RT1 and RT2 values are currently fixed from the EVM of this device. We might temporarily have to fix the thermistor to ensure the same RT1/RT2 resistors work for that chosen thermistor. The thermistor database that we have does not have the exact values that we need to calculate the values of RT1 and RT2 like you did.

    I looked at your calculations and those are correct. Please follow the datasheet guidelines on selection of the RT1 and RT2 resistors. I will work on incorporating this calculation into the tool but it might take some time before we get it up and working. However, I shall let you know once this is completed. 

    Regards,

    Amod

  • 0 in reply to Amod Vaze
    Prodigy 20 points
    Thank you.
    Minh