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BQ76940: Strange frequency noise of bq76940 with bq78350-R1

Min Kim2
Prodigy 20 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ78350-R1

I have BMS design using bq78350-R1 + bq76940, LFP 15S configuration.

I have observed strange frequency noise from the cell at  top and bottom of each ADC bank of bq76940.

            such as CELL #5, CELL #6, CELL #10, CELL #11 only, 

                       Not on CELL #1 or CELL #15 or others,

Noise level is various some channel is 30~50mV, some channel up to 200mV.

I have attached some graphs,

Can I get any advice about such a problem ?

Thanks

P/S : I have designed similar with EVM design from TI

  • 0
    TI__Guru 54715 points

    Hi Min,

    A disruption on 2s intervals such as your data shows may be an indication of a power connection problem at VC5X and VC10X.  On the EVM the power filter resistors and capacitors are on a separate schematic sheet from the IC and are easy to overlook. When the thermistors are biased for measurement the group power pins VC5X and VC10X will move up or down slightly depending on which thermistor is biased.  This normally does not affect measurement.  If the power filter resistor is not connected properly the inputs can be loaded resulting in measurement errors.  The diodes shown in the data sheet from the top cell of the group to the group power pin should be conventional diodes to allow the minor movement of the power pin without conducting.

    You can view this video that walks through the schematic creation process to help avoid common issues: https://www.youtube.com/watch?v=B5dsIc2C_xA .  The "Top 10 design considerations" appnote referenced in the video and available in the BQ76940 product folder, or other design references may help clarify the schematic connection.

  • 0 in reply to WM5295
    Prodigy 20 points

    Hi,

    Thank you for your answers and explanation.

    It is really helpful, I just check the filter resistor on VC5x/VC10x and I found the design issues.

    and, I have fixed manually, and confirmed the problem has been solved.

    Really thank so much.

    Min Kim