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BQ76930: What is the Coulomb Counter measurement bandwidth?

Chris Mayo
Prodigy 50 points
Part Number: BQ76930

In my application the load is a DC motor which starts and runs for short bursts. This results in a high peak starting current (3ms rise time, 40ms fall) and then a short run time (Typically ~200ms to 1s). I see from the datasheet that the coulomb counter produces a complete reading every 250ms in ALWAYS ON mode, but it is not clear how effective it will be at integrating the pulse waveform I have described.

Is there a figure for the effective bandwidth of the current integrator? Or putting it another way, what is the minimum pulse width that will be captured?

Many Thanks

  • 0
    TI__Guru 54715 points
    Hi Chris,
    The RC network at SRN and SRP will filter the signal. The CC runs at 256 kHz (4 us sample) as shown in the bq76930 data sheet and described in the application note www.ti.com/.../slua775.pdf. With 4 updates per second you might say the bandwidth is 2 Hz, but the CC is not trying to replicate a signal, just integrate the sampled current.
    The input filter may be the most limiting for your waveform. You might look at it as removing short pulses, or as spreading the short pulse so it can be observed by one or more samples. The single ended caps limit transients into the pins, if not a concern in your system you might reduce these. 200 ohm and 0.1uF give about 8 kHz bandwidth.
  • 0 in reply to WM5295
    Prodigy 50 points

    Hi,

    Thanks for the reply. I couldn't (still can't) find the fundamental 250kHz sample rate mentioned in the data sheet. Also I was not sure whether the charge counter uses the voltage to frequency converter and counter method I have seen on some devices, or an ADC with digital integration.

    The 250ms update rate will not be an issue for my system, provided the integrator captures the envelope of the pulses during that time.  As you say, it sounds like with the right input filter components, I should be able to capture the waveform with a reasonable degree of accuracy.

    Thanks again.