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BQ34Z65x dual I2C buffer topology

Tim Anderson76
Prodigy 130 points
Other Parts Discussed in Thread: BQ34Z651, TCA9517A, TCA9509

I am using a BQ34Z651 gas gauge, and I have two communication paths. I would like to use two 9517A type buffers. I see warnings in the datasheet that the B side isn't compatible with other B sides, and I don't know what will connect out side of my battery (it just has to be SMbus compliant). I take this to mean that I have to have the A side connected externally, and the B side connected internally (to gas gauge).

The issue, is that I need to connect two buffers to the gas gauge. Only one buffer will be in use/ enabled at one time. And they do not have to talk to eachother. They will both be connected to the same Vcc however.

If the buffers are not enabled at the same time, can I connect two B sides to the gas gauge? or will one B side screw up the other even though it is disabled?

Thanks

 

  • 0
    TI__Genius 16510 points

    Hello Tim,

    You are correct, that the TCA9517A has two different buffers (A side and B side buffers). B side buffers may not be connected to each other because signals that come from 1 TCA9517A will not propagate through the other TCA9517A.

    Another point to keep in mind is that VCCA must be less than VCCB. If this is not an option, we recommend that you look at the TCA9509. It is similar to the TCA9517A, but the A sides cannot be connected together.

    Example: The following setup will NOT allow signals to propagate through from point 1 to point 2, because the second TCA9517A will not recognize the low signal from the first TCA9517A.
    Point 1 ----- (A) TCA9517A (B) --------------------- (B) TCA9517A (A) ------ Point 2

    If I am understanding your question correctly, you are looking to do something like the below:

    | Gas Gauge | _____________ (B) TCA9517A (A) ----------- Point 1
    |BQ34Z65x   |        |_________ (B) TCA9517A (A) ---------- Point 2


    If you do not care that Point 1 cannot talk to Point 2, then the above example should be ok, provided that only 1 buffer is enabled at a time. Otherwise acknowledge bits will be sent over the other bus, or any read data from the slave (which will likely cause issues since SCL will remain high and these changes will look like start and stop conditions).