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BQ40Z50-R3-DEVICE-FW: SMBus communication circuitry design

Jack Shen2
Expert 4495 points

Part Number: BQ40Z50-R3-DEVICE-FW

I would like to check the SMBus communication circuitry. 

In bq40z50-R1, the reference design was using 100ohm and 200ohm in series for both SMBD and SMBC. 

In bq40z50-R2, the reference design got changed to use only one 100ohm resistor on SMBD and SMBC. 

May I know the following: 

1. Why does the design changed? 

2. Which one should the user use? 

3. Is there any concern if removing those resistors on SMBD and SMBC lines, and use TVS diodes instead of zener diodes? 

Thanks. 

  • 0
    TI__Guru** 105760 points

    Hello Jack,

    Those resistances are used for current protection, depending the capacitance on the lines they may need to be reduced. 100Ohm in series is pretty standard for communication lines.

    I think they should use 100Ohm in series for user applications because we don't know the capacitance on the line.

    I would also leave some series resistance, but I think it should be fine to switch the diode to TVS.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Expert 4495 points

    Thank you, Wyatt. Got few questions. 

    1. Does TI recommend to follow the reference design for SMBus circuitry in bq40z50-R1 or bq40z50-R2? 

    2. Why is there no bq40z50-R3 datasheet? Should we refer to bq40z50-R2 datasheet? 

    3. What is the current rating for SMBus pins in bq40z50? assuming there is an inrush current coming from the system. 

    Thanks. 

     

  • 0 in reply to Jack Shen2
    TI__Guru** 105760 points

    Hello Jack,

    It depends on the capacitance on the lines for how much series resistance the line can have. 100ohms should be sufficient.

    The R3 is only a firmware change.

    The pins on the gauge are rated for any current that would be caused from the voltage on the pin within the Absolute Max Ratings (AMR)

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Expert 4495 points

    Hi Wyatt, 

    Thanks for explaining that how much series resistance the line can have will depend on the capacitance.

    1. May I know what's the capacitance on bq40z50 EVM so that it uses 100 ohm resistor in series? 

    2. Do you have the formula to calculate resistance needed for different capacitance on the line? 

    Thanks.  

  • 0 in reply to Jack Shen2
    TI__Guru** 105760 points

    Hello Jack,

    I'm not sure if there's a specific standard used, I couldn't find a TI app note explaining the series resistance on the bus line. I found this exchange which may be helpful. electronics.stackexchange.com/.../series-protection-resistors-on-i2c-lines

    Sincerely,

    Wyatt Keller