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Original question:

BQ40Z50: Why is bq294700 usage suggested?

BQ40Z50: Proper SMB connection

Lukasz Przenioslo
Expert 2630 points
Part Number: BQ40Z50

Hello there,

I have a question regarding the proper SMB connection with the BQ40Z50, that at the same time needs to be able to communicate with either the host MCU on board, or the externally connected EV2300 tool. I have drawn this crude schematics to depict my case:

It is based on the eval board, to which I added the master MCU with pull up resistors. My question is:

  1. Will such config work correctly, meaning, the MCU or the EV tool will be able to communicate with the gauge with no issues (not at the same time, but one at a time, while the other stays connected to the bus)?
  2. Do the pull up resistors (pulled to the MCU VDD level) need to be disconnected when the EV2300 is connected?
  3. Should the pull up resistors be connected to the BQ40z50 VDD level instead? But on the other hand, the EV2300 tool does not care.

I would appreciate all feedback!

  • 0
    TI__Guru 57747 points

    Hello Lukasz,

    1. Communication should work in this case. However having 2 pullups could cause cross currents is the levels are different and this could impact EV2300 or MCU adversely. This is something you will need to verify.

    2. Ideally yes, but if the cross currents are within limits for both, then you can have both pullups connected all the time.

    3. Pulling up to battery voltage is not recommended because most microprocessors have absolute max ratings that are much lower

  • 0 in reply to Shirish
    Expert 2630 points

    Hi Shirish,

    1. Communication should work in this case. However having 2 pullups could cause cross currents is the levels are different and this could impact EV2300 or MCU adversely. This is something you will need to verify.

    Noted, thank you.

    3. Pulling up to battery voltage is not recommended because most microprocessors have absolute max ratings that are much lower

    Understood.

    2. Ideally yes, but if the cross currents are within limits for both, then you can have both pullups connected all the time.

    Here question is, how to figure out the maximum cross currents, especially for the tool? 

  • +1 in reply to Lukasz Przenioslo
    TI__Guru 57747 points

    Hi Lukasz,

    You can take the tolerances of the pullup V/R and get max/min current, then check what sink current the power supply design can handle. Sourcing is usually not a problem because the currents are so small.