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SN74LVC2244A - Max resistance value for unused input pullup?

Bryan Wallace
Prodigy 70 points
Other Parts Discussed in Thread: SN74LVC2244A

I have studied SCBA004C.

I have a 10K pullup on an input to an SN74LVC2244A on a board. VCC is 3.3V.

This input is unused by this board, but the signal is used by other boards in the system. There are pullups also on other boards.

In the system, the driver is an open collector. The combination of multiple pullups are causing the signal to never reach a reliable logic low, and they cannot be changed.

To protect the device when testing the board standalone, I do not want to remove the resistor completely.

I have read that cmos can be pulled up with very large resistor values.

The max input current is +/-5uA, so it should be able to be quite large.

What is the maximum resistor value I can use for this pullup?

Thank You

  • 0
    Guru 243180 points

    When the input is actually unused, the resistor value does not really matter; you could use gigaohms.

    However, SLVA485 says that

    there are disadvantages to selecting the largest allowed pull-up resistor. Larger resistances create a higher impedance net which is more susceptible to picking up noise from other nearby signals on the board that happen to couple to it. This is especially a concern for lengthy open drain outputs routed over a long distance.

    A second concern for large resistor values is from parasitic capacitance on the open drain output. This can come from other open drain outputs OR’d together, from the downstream input pins, or from nearby traces that create a capacitor in the board. This high capacitance creates an RC circuit that has an associated rise time and fall time. Final circuit operation should be validated with these longer than normal rise and fall times.
  • 0
    TI__Intellectual 1235 points

    Bryan,

    In order to meet the required Vih at the input of the device in the presence of 5uA of input leakage current, a pull-up resistor below 260k is necessary. I would suggest using a 100k or 150k resistor so that you have some headroom for Vih.

    What is your collector sink current spec? In most cases, the collector current capability of the driver should be able to easily pull the signal down to a valid logic low level, even if you had 4 or 5 of these pull-up resistors effectively acting in parallel.

    Skyler

  • 0 in reply to Clemens Ladisch
    Prodigy 70 points
    Thank you for that information. If the resistance value I need to keep enough Vil margin is too high, this may just cause more issues. I may need to remove the resistor.
  • 0 in reply to skyler
    Prodigy 70 points
    Skyler,

    Thanks for giving me a better clarification on a realistic value.

    I've been studying the complete circuit more since I originally posted. Unfortunately, there is a 10K resistor between the open-collector and the pullups to the buffer input pins, and this is not something I can change. This requires an abnormally high pullup resistor value in order to achieve a valid low (Vil) with margin in the system.

    Realistically, the resistance I would need to change this to is so high that I think I am better off allowing it to float on this board. In test and in the system it is always driven, never allowed to float.

    Thanks for all of your assistance,
    Bryan