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# BQ51050B: %Vcold and %Vhot design

Part Number: BQ51050B

Hello,

I am working with this device while I have some questions regarding the temperature sense circuit design.

If you look on the datasheet, on Page 8, there are some thresholds in percentage of internal TS bias voltage for different temperature levels, like 0, 10, 45, and 60 Celsius degrees.

I have questions regarding the equations on Page 24, how could you design the %Vhot and %Vcold thresholds?

From I understanding of this, if we say the safe charging temperature range is 0 to 60 Celsius degrees, according to the electrical characteristics on Page 8, the percentage should be typically from 13.1% (60 degrees) to 58.7% (0 degrees), right?

However, I calculated back by the given example on Page 25, and I just found out the %Vcold is 36.424% while %Vhot is 33.169%. This is quite different from the design perhaps.

Therefore, I wonder if we should define the %Vcold and %Vhot, is it free to choose in between 13.1% and 58.7%? I personally doubt a little bit about the correctness of R1 and R3 on Page 25. Would you confirm that please?

Best regards

• Weinan -- I ran the numbers and got the same thing. Input everything into Excel following the example (to validate formulas before using my target numbers. %VCOLD = 0.364, %VHOT = 0.332.

I'm going to try just targeting the %s of VTSB with my desired targets, assuming datasheet math is off for the example.
• In reply to Krunal Desai:

Hi Krunal, thanks for your reply. So I guess there should be some errors in the datasheet math. I think we should use 58.7% as %VCOLD, and 13.1% as %VHOT, and do the calculation again, do you agree?
• In reply to Weinan Ji:

I made a spreadsheet to run some values -- with a NTC of 10K and a Beta of 3375, I simply ended up with a R1 value of 2200 to reliably hit ~0.6 for the VCOLD percentage (max end of its threshold) as I'm more concerned about cold in my application than hot. I no-loaded R3 in my application (left it set to 1E9 ohms in the spreadsheet to basically null it out without having to muck with formulas).

Since it's a ratiometric divider, I didn't bother sweeping VTSB itself across the datasheet values, but sweeping the R2 values across the range also convinced me to put the 2.2K resistor in for R1 to keep me safe at cold.

Hopefully a Rev. F of the datasheet clarifies / corrects the math they have there.

• In reply to Krunal Desai:

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.

www.ti.com/lit/zip/sluc629

Best Regards,

Bill Johns,  Applications Engineer

Texas Instruments Inc

Dallas, Texas

• In reply to Bill Johns:

Hi Bill,

Thank you very much for your solution, and sorry for the late reply.

We've recently changed the battery, so we have to make the calculations again. However, the excel file seems like linking to a executable file, which is missing.

Could you please try with Step number 3 on Solver for R1 and R3 on another computer? I guess it is the library or project missing there.

Best regards

Weinan Ji