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Original question:

TIDA-010208: Diode at PACK+,- pins

TIDA-010208: Optimizing resistor selection for PACK and LD sensing

Harry McNally
Intellectual 430 points
Part Number: TIDA-010208
Other Parts Discussed in Thread: BQ76952,

Hello

I like the PACK and LD sensing circuit Ryan Tan has used (whole Sch uploaded) but I have changed to a 64V bidirectional TVS across the PACK terminals. By adjusting R141 and R142 to 1/4W, I believe the circuit will handle continuous charge voltage reversal (to -65V) so safety does not rely on current limiting on the charger.

My question is:

The data sheet shows effective input resistance of LD and PACK pins [R (ADC_IN_LD) and R (ADC_IN_DIV)] of 2Mohm and 600Kohm respectively.

We can calibrate away the resistance of the external sense circuit at manufacturing (single temperature) but I am interested in modelling the circuit to decide if we can select resistor tempco to act with the tempco of the divider in the BQ76952 to optimise measurement accuracy over pack temperature. This is for autonomous operation or where temperature compensation is not practical.

In another post, Matt Sunna has directed me to the pin equivalent circuit document, sluaaf2.pdf, which is a guide but I would need a graph of effective input resistance over temperature. Is this data that the design team has available ?

All the best
Harry 

HighSideMosfetsandDriver.pdf 

  • 0
    TI__Expert 3345 points

    Hi Harry,

    Seems that you are looking for a way to improve accuracy of PACK/LD voltage measurement. The idea of using the right external resister (suitable tempco) to allow same changes of external and internal resister with temperature will highly depends on the consistency of the resister. I suppose Matt will know the internal resister performance better than me. Normally high precision (for example 0.1%) and low tempco (for examples 25 PPM) may help to improve the accuracy even without extra manufacturing calibration. 

    Please click "This resolved my issue" if no more questions. Thanks.

    Best Regards,

    Ryan Tan

  • 0 in reply to Ryan Tan
    Intellectual 430 points

    Hi Ryan

    I appreciate the prompt reply.

    I was considering if I could optimize the parts while avoiding the cost of 0.1% resistors if and only if the tempco of the BQ was actually worse that 25 PPM. Because the internal pot-down is unaffected by temperature (since the divider is the same piece of silicon) it isn't sensitive to  tempco but the series external resistors do contribute to gain error over temperature. Can I match them; I don't know but exploring this.

    I will leave the post open to see if Matt can get any feedback from the design team.   

    But thank you for the TIDA-010208 reference design. I was only aware of it a couple of days ago and it has already clarified some approaches to the interface.

    I liked the diode D44 to allow LD to present current to the recommended 10Kohm driving PACK+.

    I am going through your comments on the PACK and LD nets:

    OCC recovery detection
    Charger detection in Sleep mode
    Charger wake up from shutdown
    Predischarge detection

    Is there other documentation that explains these other than bq76952.pdf and the Tech Reference manual ?

    There are some pdfs about improving measurement accuracy but they are more oriented to reasons why the BQ parts are superior rather than glue circuit design considerations. I will revisit both.

    Thanks once again and all the best
    Harry 

  • 0 in reply to Harry McNally
    TI__Expert 3345 points

    Hi Harry, 

    Thanks for your interest on TIDA-010208 and glad it helps. Please let me know if you further questions regarding TIDA-010208.

    Regarding BQ internal resister performance, I hope Matt will give more comments in your another E2E post.  

    For now, I only have TIDA-010208 user guide: https://www.ti.com/lit/ug/tiduey5/tiduey5.pdf?ts=1630665139157 which has some description on how it was designed. You may also check other TIDA-010208 related E2E posts for more discussions. 

    Please click "This resolved my issue" if no more questions. Thanks.

    Best Regards,

    Ryan Tan

  • 0 in reply to Ryan Tan
    Intellectual 430 points

    Hi Ryan

    Thank you, I had read through the guide and it shows cell voltage accuracy and pack current accuracy. I will look at other TIDA-010208 posts today.

    This is the only post I have on my BQ tempco question. Am I required to re-post on the Power Management forum or can Matt see this forum also ?

    All the best
    Harry

  • 0 in reply to Harry McNally
    TI__Expert 3345 points

    Hi Harry,

    Please re-post another E2E to ask any questions regarding BQ76952 inside circuit performance. That will go to product line and make Matt to see the question earlier. 

    I will close this E2E and please feel free to reopen it or post a new one if you any questions regarding TIDA-010208. 

    Thanks.

    Best Regard,

    Ryan Tan