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BQ25887: BQ25887 Charging Current Measurements

Oleg Kochurov1
Prodigy 60 points
Part Number: BQ25887

Hi!

Having trouble wile measureing charging current with built in ADC. Values read from registers REG1A:REG19 is approx. 20% lower then actual charging current. For example: actual current is 1.082 A, ADC-value is 0.857 A. At lower currents the difference reduses: actual 0.271, read from ADC 0.257 A.

All other parameters including Ibus current are correct.

Thanks for your answers.

  • 0
    TI__Genius 10385 points

    Oleg,

    Can you please elaborate on your measurement setup? Is this on the EVM? Where are you measuring? What equipment are you using to measure?

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 60 points

    1. Unfortunately I do not have the EVB. So this is on my own PCB.

    2. I measure current through higher battery (see the picture #1).

    3. I connected two multimeters in series: UNI-T UT61E and RIGOL DM3061. Both give me the same results (see the picture #2).

    4. BQ25887 registers and the decoded value for Ichg are on picture #3.

    It is 21% lower than measured value

  • 0 in reply to Oleg Kochurov1
    TI__Genius 10385 points

    Oleg,

    Have you measured the going across the CBSET resistor? I suspect some current may be going through that path as your VTOP and VBOT are significantly different.

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 60 points

    1. VTOP and VBOT are different because of two multimeter's resistance connected in series.

    2. I guess if some current goes through CBSET resistor than measured current would be less but not greater.

    3. I achieved some progress by connecting 0.01 uF capasitor at SNS pin in addition to 44 uF recommended in datasheet. Now the current difference is 6-7% and it is close to 0% at currents lower then 500 mA. But that is the best result for now. Combining different capasitors from 1 nF to 1 uF I never got accurate current measurements at 1 A or higher current.

  • 0 in reply to Oleg Kochurov1
    TI__Genius 10385 points

    Oleg,

    I will be looking into this today in the lab to see if I can recreate the inaccuracy on the EVM.

    Thanks,

    RIcardo

  • 0 in reply to Ricardo
    TI__Genius 10385 points

    Oleg,

    I am unable to recreate the discrepancy you are seeing. I have attached come photos of my setup. I have a question for you: does your layout have a sense resistor going into the battery?

    You might better off measuring current by that means. If not, I suggest ordering an EVM and seeing if you can recreate the issue on your side on that layout and use the sense resistor on the EVM.

    My gut feeling is that the multimeter is the issue here.

    Thanks,

    Ricardo

    BQ25887_ADC_Accuracy.zip

  • 0 in reply to Ricardo
    Prodigy 60 points 
    Thanks for the help.

Now I know that the problem is in my design. I will try to measure current with a low resistance sence resistor and try other components and PCB trace configurations.

I will write here if there are new results.