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PCA9306: Design Question

Jim CHEN
Expert 4795 points
Part Number: PCA9306

Dear Team,

We pullup SCL/SDA to 5V standby power. And we used 3.3V for both EN and Vref2.

Once the system enter S4/S5, 3.3V will be turn off, but 5V is still there. 

Is there any concern of this design?

Thank you.

  • 0
    TI__Guru 73971 points

    Hey Jim,

    This looks fine to me. When 3.3V goes low, the PCA9306 will become high impedance so there should be no problems.

    Thanks,

    -Bobby

  • 0 in reply to BOBBY
    TI__Expert 4795 points

    Dear Bobby,

    Thanks for reply.

    When EN is high, my customer found that if there’s no pull-up resistor on SCL1/DSA1, it still can work as well.

    From the waveform I provided, you can see there’s about 2V signal on host side even no pull-up resistor exist on side1.

    From datasheet, it doesn’t mention there’s internal pull-up in PCA9306, why the signal can work without external pull-up on side1?

    Does it means PCA9306 would cause the leakage voltage from side2 to side1 or side1 to side2?

    Thank you.

    Regards,

    Jim

  • 0 in reply to Jim CHEN
    TI__Guru 62920 points
    Hi Jim,

    There is not an internal pull-up in this device, but the way the translating function works is that a low-impedance switch is enabled between both sides whenever the voltage on either side decreases below the VREF1 level. This means that the switch will be active as the voltage on the VREF2 side is pulled up by its own pull-up resistance, and since the resistance through the device is small the same voltage will be visible on both sides. After reaching the VREF1 level, the internal switch will disable and the SDA1/SCL1 lines will be high impedance. It is probably still best to include a pull-up on these lines to make sure that there is a defined pull-up impedance to keep a stable high-level voltage on the lines when the PCA9306 switch is disabled.

    Let me know if this isn't clear or if you have any other questions.

    Best regards,
    Max