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BQ25750: Current measurements

Achim Wack
Prodigy 190 points
Part Number: BQ25750
Other Parts Discussed in Thread: BQ25780, INA214, , INA114

Tool/software:

Hi,

I´m designing an PCB with the BQ25780 and have some strange behaviour with the current measurement.

In my first design I realized, that the measurement on SRN / SRP is lower than the charging current measured at the battery. As an example, when the BQ25780 register gives me 710mA, I measured with a DMM 810mA.

In a second PCB design I also connected an additional current sense amplifier (INA214) at the shunt. Some result! Also the INA214 gives me 710mA :-O

For me it´s absolutely strange where the 100mA are gone...

Any idea?

Best regards,

Achim

  • 0
    TI__Mastermind 20120 points

    Hello Achim,

    Just to clarify, are you measuring the battery current from the BQ25750 ADC registers or are you measuring the voltage across the SRN/SRP pins or directly across the sense resistor (R24)?

    Keep in mind that this is a difficult measurement to take. With 810mA across the battery sense resistor, there would only be a 4mV difference across the resistor.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 190 points

    Hi Ethan,

    I measure by reading the registers and by measuring on the shunt resistor (SRN / SRP) with an current amplifier (INA114, U12 in my schematic attached).

    I´m aware, that the voltage on the shunt is very low, thats the reason why I use the INA114. And as difficult it might be to measure such a low voltage, I´m wondering, that the registers and the INA gives me the same current, but lower than the ampmeter in the batterie cable.

    5355.Charger.pdf

    Best regards,

    Achim

  • 0 in reply to Achim Wack
    TI__Mastermind 20120 points

    Hello Achim,

    Just to clarify further, are you measuring at SRN/SRP or at P_OUT/BAT_F? Measuring the current at SRN/SRP is different than measuring at P_OUT/BAT_F.

    It may be better to measure directly across the sense resistor for a more accurate measurement of current.

    Also, keep in mind that the BQ25750 ADC does not have an accuracy specification.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 190 points

    Hi Ethan,

    as you can see in my attached schematic I´m measuring with the INA214 across the shunt between SRN / SRP. And I did this because you told me several weeks ago in another discussion here that there´s no defined precision of the current measurement in BQ25750. But I get nearly the same results with the additional INA than from the BQ registers.

    Why is Current at BAT_F different to the measurement on the shunt at SRN/SRP?

    Best regards,

    Achim

  • 0 in reply to Achim Wack
    TI__Mastermind 20120 points

    Hello Achim,

    The voltage being measured directly across R5 will be slightly different from the voltage measured across the SRN and SRP resistors. This is because the SRN/SRP pins have an RC filter attached to them that helps the IC measure the voltage of R5.

    To summarize the two tests you've done, you've used 2 different current sense amplifiers to measure the difference across the SRN/SRP resistors. Do you get different results measuring the current directly across R5?

    Best Regards,
    Ethan Galloway