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Original question:

BQ25756: Any hints on the design changes required to operate this part in forward mode only?

BQ25756: Is there a way to share one NTC sensor between BQ25756 charger TS pin and BQ34Z100-R2 fuel gauage TS pin?

Ali Al-Sheikh
Prodigy 150 points
Part Number: BQ25756
Other Parts Discussed in Thread: BQ34Z100-R2,

Tool/software:

Hi team,

Our product has both BQ25756 charger and BQ34Z100-R2 fuel gauge. Our battery pack only comes with one NTC sensor. Is there a way to use this single sensor to allow both the charger to implement JEITA protection and the fuel gauge to report properly? I am aware that each of these ICs has a different internal regulator and that sensing levels might possibly be different. Hence seeking a creative and well-tested way to do it.

Or is it best to use two separate NTC sensors connected to the battery pack?

Let me know your thoughts!

Cheers,

Ali Al-Sheikh

  • 0
    TI__Mastermind 31420 points

    Hello Ali,

    I'm looking through the datasheet of the BQ34Z100-R2 fuel gauge and I think using two separate NTC sensors would be the best.

    The BQ34Z100-R2 TS pin works by applying a 2.5V signal directly to the thermistor and the TS pin input voltage ranges from 0V to 1V. The BQ25756 measures the temperature by using the thermistor as part as a voltage divider and the TS input voltage range is 0V to 5V. The measurements are very different between these two parts.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 150 points

    Hi Ethan,

    In this case, is there any solution you can help me with that would allow sharing some control signal(s) between the BQ34Z100-R2 fuel gauge and the charger BQ25756 to handle JEITA charging (temp controlled) yet still allow the fuel gauge to predict SOC using the temperature data, all using one NTC only? 

    • My idea so far: use the internal temp sensor setting on the BQ34Z100-R2 (which I haven't tried yet) to predict battery SOC since the whole system is sitting in a plastic enclosure, including the battery. I imagine this would only neglect the sudden battery temperature rise during loads and fast charges (stress time), which I imagine should not skew SOC prediction significantly. This way, we can allow the charger alone to access that TS and perform temp-controlled charging without issues. Does that sound like a good implementation?

    Let me know if you still think I should use two separate TS NTCs in this case.

    Cheers,

    Ali

  • 0 in reply to Ali Al-Sheikh
    TI__Mastermind 31420 points

    Hello Ali,

    Yes, I think this implementation should work for the BQ25756. I'm not as familiar with BQ34Z100-R2 IC though. Let me loop in the battery gauge experts for their input.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 37761 points

    Hello Ali,

    I believe your implementation sounds good on one condition. I would recommend running test with the internal temperature setting enabled and seeing how well the temperature is tracked based on the thermistor connected to the charger IC. If the temperature is tracked relatively closely then this implementation will be fine. If temperature is not tracked closely then this can cause gauging inaccuracies.

    Regards,

    Adrian

  • 0 in reply to Adrian
    Prodigy 150 points

    Thank you, Adrian. Makes sense. 

    Could you clarify what you mean by "relatively closely" here? Are we talking about some 1, 2, 5, 10 degrees? What number are we aiming at as a max delta here?

    Cheers,

    Ali

  • +1 in reply to Ali Al-Sheikh
    TI__Mastermind 37761 points

    Hello Ali,

    Sorry for being vague. I would aim for a max delta of 5 degrees.

    Regards,

    Adrian