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TCA9617A: pull-up and series resistor

Qianchun Liu
Prodigy 10 points
Part Number: TCA9617A
Other Parts Discussed in Thread: TCA9803, INA226, UCD9090A

TCA9617A I2C pull-up and series resistor.pdf

Hi.

Like this case，pull-up and series resitor，I think the offset voltage will be higher. Whether there are better recommendations and simulation data.

Thanks

liz

  • 0
    TI__Mastermind 41946 points

    Hey Liz,

    This device by default has an offset voltage when outputting a LOW (VOL) of 0.53V. If this is a concern for your system requirements, there are other I2C buffers that can be used such as the TCA980x series (also supports VCCA= VCCB as in your application). These I2C buffers rely on an internal current source instead of an offset voltage to realize which side of the device is driving low, and allow for a smaller VOL (i.e TCA9803 max VOL = 0.26V). However, note that if this device is used, pullups/ series resistors cannot be used on B-side of the device. 

    In order to accurately see if the pullup resistors will impact the offset voltage higher, the IOL values are needed. Please see this app note  (specifically page 2) and this similar E2E question for more information.

    Regards,

    Jack 

  • 0
    TI__Guru 50048 points

    Hi Liz,

    I don't see any major problem in the schematic. I would like to note that the MUX/DEMUX in use is probably not a true buffer. This means that each side will not truly be separated when in a LOW logic state. Pull-up resistors and series resistors will combine from the B-side to the A-side. 

    For example, the pull-up resistors: 2.2k || 2.2k || 10k = ~990.99 ohm. This combined pull-up resistor value is fine, but any stronger and you might run into VOL being too high of a voltage. 

    The series resistors may also combine: 33 + 33 + 33 = 99ohm of series resistance at UCD9090A and INA226. I would double check to ensure that this series resistance will not interfere with transmitting valid I2C signals. 

    If we want to study more on whether this setup will work, we can look at the scope captures to see if the I2C signals are clean and robust. 

    Regards,

    Tyler