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Firmware / Current Sensing for BQ78PL114

Jonathan Wright
Prodigy 40 points
Other Parts Discussed in Thread: BQ78PL114, BQWIZARD

Hi,

 I am working within a team designing a Li-Po battery management system using the BQ78PL114 and I was wondering if you could clear a few things up for us.

We are having problems designing the current sensing section within the system and were wondering:

  • What firmware is installed within the 114 as standard (FW4452 or FW5000)?
  • the data sheet says that FW4452 permits the use of a 5 mOHM sense resistor.... is this flexible ie. can it be a smaller resistance?
  • Do you have any suggestions regarding the construction of the sense resistor ?

Thanks for your help

Jonathan

  • 0
    TI__Guru 73830 points

    Jonathan, 

    The bq78PL114 is shipped with 4452 firmware. The user can update the device to the bq78PL114S12 by loading the 5000 firmware. The 4452 firmware version is tuned to use a 5 mohm sense resistor. The 5000 firmware version offers the flexibility to change the sense resistor using the PackConfigurationUtility in bqWizard.

    The construction of the sense resistor will be dependent on several factors including value, power rating, availability, etc. We cover this, among other things, in the bq78PL114 System Design Guidelines Application Report.

    Regards,

    Tom

  • 0 in reply to ThomasCosby
    Prodigy 40 points

    Thanks for the reply. We are having a few other issues with the sense resistor:

    We are designing a high current system that will peak at 180A for 1 second and 80A for up to 10 seconds. (for turning on the motor). With a 5milliohm resistor the voltage input at the BQ78PL114 chip will exceed the maximum voltage ratings for the current sensing circuitry. Will this damage the chip or will it just cause an error in the reading?


    As an alternative we are considering using a hall sensor to measure the current through the power lines, however, will we need to be calibrated  the hall sensor to suit the chip?

    Would you advise updating the firmware so that we can use a 1miliohm sense resistor instead? We have already got all components for the 4452 firmware, will the update change or design? Alternatively, would you recommend us using a hall sensor? Or is it possible to keep the design with a 5 miliohm resistor and just have an error when the voltage is too high? Otherwise, can you suggest a specific component or solution to our problem?

    We are students doing a university project and we would really appreciate your help with this problem!

    Thanks!

  • 0 in reply to Jonathan Wright
    TI__Guru 73830 points

    Jonathan,

    The excess current probably will not physically damage the device, but the measurement system may be affected. You will also need a very high power rating on your resistor. I recommend updating to the PL114-S12 or the PL116 and size the measurement system for your application.

    I have not dealt with hall sensors in this type of application.

    Regards

    Tom