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TXS0104 vs TXB0104: Push-Pull versus Open-Drain Support

user1896237
Prodigy 20 points
Other Parts Discussed in Thread: TXB0104

We use a level translator to shift 5V to 3.3V between a device and MCU in I2C and SPI bus. But I found the communication is failed. When I changed the TXB0104 to TXS0104, it comes to work. I notice the datasheet note that TXB0104 is not suitable for OD applications. I think the debug of my project fully proved this. My question is what's the difference between these two chips and why TXB0104 is not suitable for OD applications(seems the 4kohm res on the output side contrubute)? And why push-pull or OD device affect the level translator? Looking for your reply. Thank you in advance.

  • 0
    TI__Expert 6335 points

    Let's start with discussing an open drain interface:

    To produce a Logic High in an open drain interface, a pullup resistor to the supply voltage is required. An example for I2C would be an 8k pullup resistor to 3.3V. This produces a Logic High of approximately 3.3V on the bus.

    A pulldown resistor on the bus results in a voltage divider. An example for I2C would be an 8k pullup resistor to 3.3V AND a 4k pulldown resistor to GND. This reduces the Logic High on the bus from approximately 3.3V down to (3.3V)/(4k/12k) = 1.1V. This 1.1V is typically not large enough to be recognized as a Logic High by most I2C devices.

    The impact of the TXB0104 is similar to the 4k pulldown resistor discussed above:

    The output buffer of the TXB0104 has output impedance of 4k. When the TXB0104 drives the bus low, the output buffer resembles a 4k pulldown resistor to GND. So even if an external device releases its hold on the bus (to let the external pullup resistor pull the line up to 3.3V), the TXB0104 will hold the bus low.

    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    Prodigy 20 points
    Thank you Nirav, I think I got your answer. Even if an external device releases the bus, the one-shot in TXB0104 cannot recognize the low-to-high transition(the voltage on the bus is not large enough) so it will still turn on T2 and T4 which makes the 4k res resemble to GND. Is is correct?
  • 0 in reply to user1896237
    TI__Expert 6335 points
    Almost.

    When the one-shots (T1, T2, T3, and T4) are activated, they remain enabled for approximately 10 ns, and then turn off. After that 10 ns expires, the output buffer (with 4k output impedance) continues driving the line to either Logic High or Logic Low, but with a much slower rise/fall time than the one-shots. The one-shots are active during signal transition whereas the output buffers are active during signal steady-state.

    Normally when driving high, T1 and T3 are enabled for 10 ns. With a pulldown resistor however, T1 and T3 are not enabled due to the 4k output buffer holding the open-drain 'release' voltage below the T1 and T3 voltage threshold.

    Best Regards,
    Nirav