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BQ76952: BQ76952 for supercap monitoring

Mike Twieg1
Intellectual 390 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQ76940, BQ79616-Q1, BQ79616

Hello,

I want to measure cell voltages on a 14s stack of supercaps. The supercap-specific devices from TI don't seem to support more than 8 cells, so I was considering the BQ76952. I do not need it to control charging, or balancing, or protection, just monitor cell voltages and provide UVLO/OVLO alarms to a MCU. Was also looking at the bq76940, but its UVLO/OVLO detection thresholds aren't suitable for supercap cells, I think.

Anyways I had some questions about using the BQ76952 for this application:

1. At times the pack voltage will be near zero, but I still want the BQ76952 to be powered and communicate with the MCU. Therefore the BQ76952 should not be powered from the pack itself, but another DC supply on my board. Is there any reference design for something like this?

2. What sort of conversion speed is achievable for all cells in the pack? Section 10.1.1 in the datasheet mentions a "loop" which converts all 16 cell voltages, but then says "Over the course of three
loops, a full set of measurements is completed". Why three loops?

3. Ideally I'd like the timing of these samples to be synchronized to other events in the system (mainly for self testing purposes). Is it possible to manually trigger a loop?

4. Does TI offer any other options for measuring cell stack voltages? Maybe something similar to the MAX14921 from Maxim/Analog?

Thanks,

Mike

  • 0
    TI__Guru* 81034 points

    Hi Mike,

    1. The BAT pin could be powered separately. There is not a reference design for this, but it should be straight-forward to power BAT from a different source.

    2. The cell voltages are measured on a 63 ms loop. This is described best in this technical article: https://e2e.ti.com/blogs_/b/powerhouse/posts/improving-voltage-measurement-accuracy-in-battery-monitoring-systems The three loops are needed to complete other measurements and thermistor readings.

    3. There are synchronized voltage and current measurements - these are available in the DAStatus registers and are described in the BQ76952 TRM. It is not possible to synchronize measurements to an external trigger though. You could use the Alert pin of the device to signal to the system when measurement loops are completed.

    4. For devices more similar to the product you mention, you may want to take a look at the BQ79616-Q1. I do not think you will need this type of device for the application described though.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Intellectual 390 points

    Hi Matt, thanks for the quick reply, my replies are below:

    1. Sounds good. Just to clarify, I will have to make sure REGIN is provided 5.5V, therefore BAT will need to be at least 5.5V as well? I do not intend to make use of REG1 or REG2. Are there any other bias voltage limitations to watch out for? I do not intend to use the OTP, or the charge pump.

    2. Ok I think I understand. Bottom line is that the cell voltages will be updated every loop (63ms, depending on other configs). Some other measurements may take up to three loops to update, but that's fine with me.

    3. Ok, if the cell voltages update every 63ms then the synchronization is probably not critical.

    4. The BQ79616 also looks like a candidate (though it's very different from the maxim part I mentioned). Even faster cell measurements too. At a glance, it the communication interface looks more complex... but will look more at this. 

  • 0 in reply to Mike Twieg1
    TI__Guru* 81034 points

    Hi Mike,

    1. Yes, 5.5V on the BAT pin will work since this meets the minimum recommended voltage for the BAT pin. If REGIN is not needed (it is not needed if you are not using REG1 or REG2), REGIN can be connected to VSS. 

    Best regards,

    Matt