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TXS0108E: Output level issue with Pull-Down resistor

user6524897
Prodigy 20 points
Part Number: TXS0108E
Other Parts Discussed in Thread: SN74AHC541, SN74AHC125

Hi Team!

Currently we are designing our PCB with the TXS0108E. As we also have some external limit switches with 5V supply voltage, we would like to connect them to our 3V3 MCU via the level shifter. Therefore we tested it with a 1M Pull-Down resistor on one of the B input pins and also with 5V supply voltage. Unfortunately we were not able to get the desired output level while changing from no input voltage (GND via Pull-Down) to 5V. We also have a question regarding the OE input pin: Is it necessary to also have there a Pull-Down resistor or can it directly be connected to the A port voltage level of 3V3? We also noticed that there might be an issue with the Pull-Down resistor value at the signal input pin if it is too low but we think we chose it high enough. Attached is the schematics. Thanks in advance.

  • 0
    Guru 287370 points

    The TXS has internal pull-up resistors. If possible, put the switch between the input and GND. Otherwise, you'd have to use a much stronger pull-down resistor, which would waste power.

    The unused pins should not be grounded; see [FAQ] What should be done with unused I/O pins of the level translator devices?

    The pull-down at the OE pin is necessary only if you want to set it high some time after power up.

    How many switches? For converting the signals to a lower voltage, you could simply use a buffer with overvoltage-tolerant inputs, such as the SN74AHC125 or SN74AHC541.

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Thank you very much for the fast reply!

    Are there some estimated values available for the pull-down at the OE pin?

    Currently we are planning to use 4 limit switches with a total cable length of 12m per switch. Could the cable length be a problem for the input? We also would like to use I2C as well on the same IC. So we think the TXS0108E would suit.

    Best Regards

  • 0 in reply to user6524897
    TI__Mastermind 44905 points

    Hi,

    The OE pin doesn't need a resistor, feel free to direct connect to Vcc(A or B). This device does not work well long cables (typically adds a large capacitance and its recommended to be <70 pF). I'd recommend switching to a different device like the ones suggested by Clemens.