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TXB0304: TXB0304 pull-up or pull-down resistors

Olin Zhang
Prodigy 630 points
Part Number: TXB0304

Hi team,

My custmoer uses TXB0304 with a baud rate of 962kbps. The datasheet suggests that the pull-up or pull-down resistors must be kept higher than 20 kΩ to ensure that they do not contend with the output drivers of the TXB0304, but the customer used the 1kΩ pull-down resistor and it  can also meet the requirements.

So the customer raised three questions:

1. What is the difference between using 1kΩ and 20kΩ pull-down resistors, and what effect it has on the use of TXB0304;
2. What is the value of the internal pull-up and pull-down resistors of the I/O port. If it is 1kΩ, how can 1.5kΩ in the data sheet be obtained?

3. If the test result meet the requirement, whether the 1kΩ resistor can guarantee the reliability of the TXB0304 converting signal in all scenarios.

Would you please give your comments? Thank you!

  • 0
    TI__Guru* 97046 points

    1. What is the difference between using 1kΩ and 20kΩ pull-down resistors, and what effect it has on the use of TXB0304.

    The TXB0304 has 1kohm series output resistors during static operation. Putting a 1kohm pull-down will result in about 1/2*Vcc at the output, which is extremely undesirable in the vast majority of CMOS circuits. Not only will this cause problems for external devices, it will cause problems for the TXB0304.

    The output voltage can be quickly estimated using the voltage divider equation: Vo = Vcc * Rpd / (1k + Rpd)

    2. What is the value of the internal pull-up and pull-down resistors of the I/O port. If it is 1kΩ, how can 1.5kΩ in the data sheet be obtained?

    Resistors on semiconductor substrate (like those used in the TXB0304) are not as precise as external resistors. They have a fairly wide margin of values that they can be depending on temperature and process. 1.5kohm is used here to provide margin for the worst-case. Adding additional pull-down resistors to this circuit is generally a bad idea. I would recommend taking a second look at the system design rather than trying to work around by fiddling with resistor values.

    3. If the test result meet the requirement, whether the 1kΩ resistor can guarantee the reliability of the TXB0304 converting signal in all scenarios.

    What test result? What requirement?

    A 1kΩ resistor should not be used to pull-down the output of the TXB0304. If a 1kΩ pull-down is required for some reason, then I would recommend changing the translation device to one that is designed to operate in that scenario.