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BQ76940: Current Measurement - Precision

Mauro Basquera
Intellectual 720 points
Part Number: BQ76940

Hi,

I want to know a few things about the bq76940 current measurement.

1) Do I need to set some bit to activate the full scale range? Because in the datasheet it says that the CC input voltage range is +-200mV and the full scale range is +-270mV.

Calculating the LSB we get (540mV)/(2^16) = 8.23 uV. Which is pretty close to the value given in the datasheet, 8.44 uV. So I assume that the full scale range is already activated. Is there any problem in working with this range?

2) Is there any way to enhance the current measurement precision of the bq76940? What is the usual guidance for it? Our system is a 48V/50Ah with current peaks of 150A

We thought about a PGA connected to its inputs (SRN/SRP) but we don't know if it's possible and how we control it.

Thank you,

  • 0
    TI__Expert 6085 points

    The CC is 16-bit Integrator.   The I bit represent the sign negation of the current. Positive values are CHG  current; Negative  for DSG current.

    The full scale +/- 32000.  The 1 LSB of the CC 270mV)/ (2^15) = 8..23uV.   Hence  in the  recommended operation range of CC is, +/- 200mV, there are

    200mV/8.23uV ~ 24000 count.

    Also, the CC is an integrator ( 250ms) and would not be fast enouigh for fast transients or instantaneous current decisions. 

    (2) The SCD  threshold are  200mV ( (RSNS=1) or 100mV ( RSNS=0)'  so as an example  for the 150A requirement  with Rsense= 1m-Ohm( .1%) should be a comfortable range of selection. 

    Likewise OCD threshold is 100mV. 

    there is step procedure in the d/s in section 8.2.2

    hope this helps,

    vish

  • 0 in reply to Vish Nadarajah
    Intellectual 720 points

    Hello, Vish.

    Thank you for the response!

    About the short-circuit condition. It's written on the datasheet that the Abs Max SRP-SRN input is 300mV.

    We still don't know the exact current of our system in this condition, but will the input be damaged if this value is exceed or there is a internal clamping circuit to protect the SCD Comparator?

    Is 300 mV the maximum value acceptable without damaging or it is another value?

    If it is not, what do you recommend for protecting the SRP-SRN input?

    Thank you.

    Regards,

  • 0 in reply to Vish Nadarajah
    Intellectual 720 points
    Hello, Vish.

    Do you have the information's?

    Thanks,
  • 0 in reply to Mauro Basquera
    TI__Expert 6085 points

    The d/s has recommend network to protect the SRN/SRP pin spurious spike. differential , the value is +/-300mV for the range of the measurements.

    SRN/SRP input ABS MAX rating is +3.6V  and -0.3.