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BQ76952: Layout for Parallel current sense resistor

Shyama Agrawal
Intellectual 860 points
Part Number: BQ76952

Hello,

We are facing with accuracy of current measurement and coulomb counting at low currents (10-50mA). We have used 3 sense resistor in parallel (1mohm each)as seen below. The voltage when measured across these sense resistors at low currents in not as expected. The current of 10-50mA is flowing through R_NT1. No current is flowing from PACK- to BAT-.

When an accumulation of 100mAh is expected at 10mA load after 10hrs, we are getting only 30mAh. We are expecting issue with sense resistor layout (Kelvin connection design) or position of Net Tie (R_NT1). Please suggest what you think.

  • +1
    TI__Mastermind 38360 points

    Hello Shyama,

    We usually recommend ~1-mOhm resistance, although for higher current applications it may be decreased. Your sense resistance is too small for accurate measurement at lower currents.

    Your equivalent sense resistance is (1/3)-mOhm, looking at Section 7.15 (Coulomb Counter) of the datasheet, we can see that the offset error of the part is ~7.6-uV. The offset error with a (1/3)-mOhm sense resistor, would give a maximum current offset error of ~22.8-mA.

    Besides the offset error, your current resolution is also decreased with lower sense resistance, the coulomb counter input range is -0.2-V to 0.2-V over an ideal 16-bits range, this is about a voltage resolution of ~6.10-uV , with your sense resistance, the current resolution becomes ~18.3-mA (this would be in an ideal scenario, the actual current resolution would be worse).

    Best Regards,

    Luis Hernandez Salomon