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bq51050b R1 and R3 Calculation

Mauricio Solis Jr
Prodigy 645 points
Other Parts Discussed in Thread: BQ51051B, BQ51052B, BQ51050B

HI,

I'm having trouble understanding the math required for R1 and R3. Can someone please clarify????

I tried going through the example on the datasheet, Page 25 and 26, but I do not understand the math for values for R1 and R3.

My process:

1.) I calculate R,NTC|COLD and R, NTC|HOT using the following...

This equates to roughly,

R, NTC|COLD = 28.224 kOhm

R, NTC|HOT = 3.039 kOhm

2.) This is where I'm confused.  Using equations 3 and 4  from the datasheet page 25, I try solving for R1 and R3.

Where R2 = 20kOhm as stated in the circuit from above.

Based on the Electrical Characteristics table from page 8, I use the values of 58.7% and 13.1% as my %V, COLD and %V, HOT respectively.

I'm not sure what to do after this.  

What I know,

If R3 is significantly larger than R1+RNTC or R1 is significantly smaller than RNTC, then one can remove R3 and R1 respectively  I do not wish to have an R3 value.  Do I just set it to 1MOhm and work the equation that way???  

Someone, please clarify. It would be appreciated if you showed the maths.

Thank you,

Mauricio Solis 

  • 0
    TI__Genius 12475 points
    Mauricio,

    We have answered the question on email. thanks for posting here!

    Regards,
    Gautham Ramachandran
    Applications Engineer
  • 0 in reply to Gautham
    Prodigy 10 points

    Hi,

    Apologies if this is reviving an old thread but could you please share the details? We have the exact same issue when working through these calculations.

    Many thanks

    Joe

  • 0 in reply to Gautham
    Prodigy 20 points
    Hello,

    Can you please share the answer regarding the computation of R1 and R3?

    With the values reported in the datasheet:

    R2= 20 KOhm
    R1= 29.402 KOhm
    R3= 14.302 KOhm

    and with the values reported above by Mauricio:

    R, NTC|COLD = 28.224 kOhm
    R, NTC|HOT = 3.039 kOhm

    The two above equations yeld the following values, that don't seem to make sense:

    %Vcold= 36.42%
    %Vhot= 33.17%

    If, instead, I solve the two above equations imposing the values that Mauricio indicated:

    %Vcold= 58.7%
    %Vhot= 13.1%

    then, assuming that the internal R2= 20 KOhm, I obtain the following values for R1 and R3:

    R1= 0 Ohm (short circuit)
    R3= 1000 MOhm (open circuit)

    What are the correct values of R1 and R3 for the indicated (10K, Beta=3380) NTC? Is the datasheet incorrect? Please help!

    Thank you,
    Maurizio
  • 0 in reply to Maurizio Banfi
    Prodigy 60 points

    Hi community,


    I am at the same point as you Maurizio Banfi. Having no clue what to make of these equations, I just solved them with the same values as were given in the datasheet and I arrived at the same values for %Vcold and %Vhot as you (36.4% and 33.2%).


    It would be great to have the meaning of "%Vcold and %Vhot" explained since these are never mentioned or explained anywhere in the datasheet.

    Your last assumption with R1 = 0 Ohm and R3 = 1000M Ohm would mean "just connect the NTC to the TS/CTRL pin" (and maybe add that C3 which is probably a noise filter?) and nothing else. Did you try that?

    I checked with the bq51050BEVM evaluation kit's schematics but I can't make any more sense of this (except that they seem to use 0.1 uF for C3 on the EVM) since they don't include a temperature sensor on the board but only a connector. There is also no sign of anything similar to the values of R1 and R3 that were calculated in the datasheet example.

    Help would be greatly appreciated!

    Thanks and best regards,

    Jonathan

  • 0 in reply to Jonathan Bischof
    Guru 21675 points
    Hi,

    Please let me know the optimal R1 and R3 value for following NTC.

    Tcold = 0 degC
    Thod = 45 degC
    Beta = 4050K
    Ro = 10kohm

    I'd like you to share the tool to calculate it if you have. Please see my profile and find email address.

    Best Regards,
    Satoshi / Japan Disty
  • 0 in reply to S.Satoshi
    TI__Guru 68796 points
    I will email you the info.
  • 0 in reply to Bill Johns
    TI__Guru 68796 points
    NOTE--R1 and R3 values in data sheet page 26 are incorrect and will be changed on next revision, in work now.

    An NTC Calculator Tool has been added to the product folder--Tools & software for bq51050B, bq51051B and bq51052B.
    The tool is an excel spread sheet that can be used to calculate R1, R1&R3 and verify results.

    See below link
    www.ti.com/lit/zip/sluc629
  • 0 in reply to Bill Johns
    Guru 21675 points

    Hi Bill,

    Thank you for the update. I'll refer spread sheet you informed to calculate R1 and R3 value.

    Best Regards,

    Satoshi / Japan Disty