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BQ76940: Adding a hardware-only protections layer to my bq76940 BMS

Sebastian de Leon
Prodigy 190 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ77905, BQ76200

Hi,

I'm designing the bms for a 13s4p pack intended for e-bikes, using the bq76940 as the monitor IC, and as the battery is quite power capable, I would like to add a complete layer of hardware-only protections.

Because cell-balancing is made by the AFE, the bq77905 would be my choice.

1. Is it a good practice to use these two chips together or am I over-engineering the design?

2. I'm using high side fets, driving them with the bq76200. Should I OR both CHG pins coming from the bq77905 and the AFE to the bq76200 (with diodes), and OR both DCHG pins from the bq77905 and AFE to the bq76200?

3. In that case, how can I use the load removal pin from the bq77905?

Thanks very much for your time and help.

Kind regards,

Sebastian

  • 0
    TI__Expert 7455 points

    Hi Sebastian,

    1. Yes this is good practice. We see this often. Adding a second layer of protection will help prevent disaster in emergency situations.
    2. Yes, this configuration could work. Ultimately, it is up to you and your application, but I think this is a good solution.
    3. The load removal might require some external circuitry to force the LD pin low when there is no load. 

    Let me know if you have any more questions!

  • 0 in reply to Shawn Hinkle
    Prodigy 190 points

    Hi Shawn,

    Thanks very much for your fast answer. My next questions:

    1. I'm reading the voltage across the sense resistor through RC filters from 3 devices, TI's full stack gauge, the AFE and the BQ77905. I'm using a 2 pin resistor. Could this create some loading that will impact on the sense resistor reading for the devices mentioned?

    2. Regarding the same RC filters, do the tolerance of the 100 ohms resistors affect the accuracy of the measurement or only pcb traces res + sense resistor tolerance?

    Thanks again

  • 0 in reply to Sebastian de Leon
    TI__Expert 7455 points

    Hi Sebastian,

    1. Yes, it is possible that could affect the sense resistor reading.
    2. Yes the tolerance does matter. A smaller tolerance will lead to a more accurate reading.