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Original question:

TCA39306: connection in star configuration

TCA39306: Series connection of devices

Harley Cumming
Prodigy 110 points
Part Number: TCA39306

Hi, I'm interested in using the TCA39306 in a series configuration. I'd like to connect a 1.8V I2C bus to a 3.3V I2C bus using a TCA39306, with multiple devices on both the 1.8V and 3.3V domains. I'd also like add a second TCA39306 to convert from 3.3V to 5.5V for another set of devices at 5.5V. Assuming capacitive loading is kept in check, is there any issue with this type of series connection of TCA39306 devices?

Thanks!

  • +1
    TI__Guru 71910 points

    Hi Harley,

    The main concern would probably be higher current (IoL) when both devices are enabled. If the 1.8V logic receiving lows from the 5V5 logic side for example, the pull up resistors from the 1.8V, 3.3V, and 5V5 would technically all be in parallel with one another. This would make the VoL on the 5V5 side larger. The series resistances from the TCA39306s would shift the VoL higher from the perspective of the 1.8V side. The flip side of this could also be considered where the 1.8V side is driving and would need to be able to sink all the current from all the parallel resistors (1.8V, 3.3V and 5V5). You wouldn't want the 1.8V I2C devices to saturate when they drive low. 

    Placing pass FET translators in series can work, I would just suggest that you don't use pull up resistors that are too strong. Just pick values that would meet the I2C rise time requirements for your required frequency and bus loading. 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 110 points

    Thanks, Bobby! I've seen that behavior in the past, and that makes perfect sense. Thanks for the clarification.