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BQ76942: Two AFE stacked, the low AFE need control N-MOSFET direct.

user5883945
Expert 1826 points
Part Number: BQ76942
Other Parts Discussed in Thread: TIDA-010247

Dears，

   

We refer to TIDA-010247 to do 2pcs AFE stack design to meet the requirements of 24-string battery applications;

TIDA-010247 can only be directly controlled by the High AFE, and cannot be directly controlled by the Low AFE. As a result, TIDA-010247 cannot immediately control the N-MOSFET when it detects a fault, resulting in delay.

We need advice on how to design the external circuit so that low AFE can directly control the N-MOSFET instead of reporting errors to the MCU or High AFE via I2C.

  • 0
    TI__Mastermind 42060 points

    Hello User,

    The low-side AFE can control the top's FETs through the CFETOFF/DFETOFF pins. The TIDA-010247 implements a circuit that will cause a high signal to be detected on the top CFETOFF pin to turn-off the CHG FET:

    This provides a hardware path to turn-off the FETs and can turn-off the FETs off very quickly. If you have any questions on this, let me know Slight smile.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Expert 1826 points

    Hi，

      1.The customer has designed a logic circuit, so that both AFEs can directly control the FET, the circuit is as follows, whether it is feasible？

    2. TIDA-010247 

    TIDA-010247 will connect two AFE I2C together, how to control its communication, will there be no conflict？

  • 0 in reply to user5883945
    TI__Mastermind 42060 points

    Hello User,

    They should analyze the circuit thoroughly to ensure it will work. However, not sure how well it will work. If they can explain their thought process that'd be good. For the CHG FET I don't quite see how M16 may be able to turn-off correctly. The M16 looks to me that it may be floating regardless of the state of the bottom driver. 

    For the DSG FET it is even more complicated, as the DSG source voltage (PACK+) may be able to drop all the way to Vss. This means that there will need to be circuitry to allow the source to drop that low and protect the DSG/LD pins of the top AFE.

    The TIDA-010247 adds circuitry to achieve that.

    Controlling the FETs with the CFETOFF/DFETOFF pins would achieve the same thing that this circuit, but much easily so. Is there a reason why they do not want to use the CFETOFF/DFETOFF pins?

    For the second questions, it depends. An ISO device is required to communicate as the ground reference would be different between both AFEs. However, for the conflict, usually you would want to change the I2C address of one of the devices. This can be done by powering the AFE's separately and program one first with a different address. The alternative would be to perform an OTP to one of the devices with a custom I2C address. 

    Best Regards,

    Luis Hernandez Salomon