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BQ25628E: Thermistor Bias Resistor question

s_breakpoint
Prodigy 50 points
Part Number: BQ25628E

Tool/software:

Hi!

I'm writing with a question about the BQ25628E battery charger IC. This is the first time I'm using this part, and my question is about the thermistor bias resistor calculations outlines on p. 24, 8.3.4.5.2. The sheet states:

"Assuming a 103AT NTC thermistor on the battery pack, the RT1 and RT2 are calculated to be 5.23 kΩ and 30.1kΩ respectively"

And there are formulas provided for this.
However, when I calculate these myself, I arrive at R1/R2 values of 5,131.82 and 27,665.36, respectively. Rounding these to e-series suggests 5.1k and, I don’t know, maybe 27k or 27.4k. 
I am curious if TI can offer insight as to why the datasheet numbers seem to differ slightly?
I'm aware that it’s not ideal to be pedantic about numbers in designs like this, but I'm just curious - as a relative newcomer to designs that include charger ICs - if TI might have any practical guidance or thoughts about how to think about these?
Thank you!
  • +1
    TI__Mastermind 31100 points

    Hello, 

    Thank you for reaching out via E2E. Please see my comments below.

    s_breakpoint said:
    I am curious if TI can offer insight as to why the datasheet numbers seem to differ slightly?

    I won't go into a detailed breakdown of the math at this time, but our calculations with the the 103AT NTC output 5.24 kohms and 30.42 kohms for RT1 and RT2. Those values are then rounded to standard resistor sizes 5.23 kohm and 30.1kohms. 

    For your reference we do have an excel based tool for calculating RT1 and RT2 with the equations embedded. You can download the file from the link below, as it should help to further analyze why your calculations differed a small amount.

    https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1209815/faq-bq25620-bq2562x-calculator-tool 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 50 points

    Ah, thank you - I think I had come across a version of this spreadsheet before. Looking through it more closely, I see what my issue is: I had been using this equation to calculate R_COLD and R_HOT:

    R(T) = R0 * exp[B(1/T - 1/T0)]

    but I see this spreadsheet uses values provided directly in the datasheet for the 103AT, which differ slightly from the calculated values. This is what apparently led me slightly astray. 

    Many thanks for helping me understand that!