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BQ76930: Question about the calculation of the sense resistor

FranMartin
Intellectual 600 points
Part Number: BQ76930

Hello,
I have a question about the calculation of sense resistor. In the datasheet, in section 8.2.2.1 'Step-by-Step Design Procedure' (on page 50). I can not correctly understand the calculation of the  resistor value. It is specified that the 'Abs Max SRP-SRN input is -300 mV'. In what section of the datasheet can I find this information or how is it calculated? In addition, the resistor value  is calculated as: RSNS = 300 mV / 25 A = 7.5 mOhm but that division is incorrect, right?

Thanks,

Fran Martin

  • 0
    TI__Guru 54715 points
    Hi Fran Martin,
    The division shown is incorrect, 300/25 is 12. It is difficult to tell the intent of the author or how the sentence may have been damaged in revision. The general idea is that there are different calculations which will set different limits on the sense resistor. The 300 mV does not make sense since abs max could be much higher. The maximum SCD threshold which can be selected is 200 mV, 200 mV / 25 A = 8 mOhm, 7.5 would be suitable here but the author considers tolerance later when selecting the 5 mOhm value.
    Basically consider the limits of the system and the available thresholds in the bq769x0 when selecting the sense resistor. Also consider the tolerance of the sense resistor and the tolerance of the thresholds. Also consider and trade off that a larger sense resistor will give a finer current resolution but more power. A smaller sense resistor will reduce the power dissipated in the battery pack but give a coarser resolution of the current.
    I will mark that sentence as in need of correction in a future document revision.