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BQ34110: bq34110: Rsense greater than 20mOhms for greater resolution

user4725373
Expert 1645 points
Part Number: BQ34110
Other Parts Discussed in Thread: BQ34Z100-G1, BQSTUDIO

Hi,

The datasheet for the bq34110 says that Rsense should be between 5 and 20 mOhms. However, the app note slua792 tells that it's not really a problem. Are there any problems with increasing the Rsense to get a better LSB for the CC ADC?

For example, I got that the worst LSB (when ADC is 14 bits, not 15) in volts of the device is:

LSB = V(SR) / 2^14 = 0.25 / 2^14 = 15.259uV

If I want a current LSB of 50uA, I need to have a Rsense of 305.18 mOhms. This sets my maximum readable current to:

I(max) = V(SR) / 2 / Rsense = 0.25 / 2 / 0.305 = ~410mA

This maximum current is perfectly fine for our application. Except for a higher dissipation at Rsense, are there any reasons why this Rsense value wouldn't be acceptable?

Thanks,

Fred

  • 0
    TI__Guru 73830 points
    Fred
    The voltage drop across the sense resistor must be kept below +/-125mV, so the sense resistor can be up to 304 mohm. We may have to provide a special .bqz file to allow this value resistor to be entered. You may also want to use reverse current scaling to allow the current reporting to use more of the register. The is an application report on the bq34z100-G1 website that covers normal scaling. Yo will scale up instead of down.
    Tom
  • 0 in reply to ThomasCosby
    Expert 1645 points
    Hi Thomas, thanks for the answer.

    As I will not use the bq34110 with bqStudio, but rather with a microcontroler, will I be able to translate the bqz file to I2C commands?

    Also, you mentionned using scaling (n_cells_register = real_n_cells / scaling_factor). Would that scale not affect the CEDV algorithm? Should I perform my CEDV characterization with 2 cells in series instead of one?

    Thank you
  • 0 in reply to user4725373
    TI__Guru 73830 points

    You will not need the bqz, if you do not use bqStudio. I think that your design process will go a lot smoother, if you do use bqStudio, You would need to scale before running the discharges to find the CEDV coefficients and you can use either the one cell or the stack of two.

  • 0 in reply to ThomasCosby
    Expert 1645 points
    Well I was going to use bqStudio for characterizing my cells, but for the calibration step and the product itself, it has to be done with an MCU.

    Thank you for your time!