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SN74AUP2G17: Input Pull-Down Resistor

Ahmed Abed Benbuk
Prodigy 30 points
Part Number: SN74AUP2G17

Hello,

I am using the SN74AUP2G17 at the input of a digital circuit. I would like to know what is the required value of a pull down resistor so that the output doesn't float?

Thank you. 

  • 0
    TI__Guru* 96947 points

    Hi Ahmed,

    I'm not sure what you mean -- the output of the SN74AUP2G17 is a push-pull driver, which means it will never be floating.

    If you're talking about the input, then a 10kohm resistor will work fine in most circumstances.

    if you want to know how to properly size a pull-up resistor, we have an FAQ on that topic, located here:

    [FAQ] How do I size pull-up or pull-down resistors? - Logic forum - Logic - TI E2E support forums

    e2e.ti.com
    Other Parts Discussed in Thread: SN74AUP1G34 FAQ: Logic and Voltage Translation > Input Parameters >> Current FAQ Pull-up and pull-down resistors are required

    Please let me know if I can be of further assistance. 

  • 0 in reply to Emrys Maier
    Prodigy 30 points
    "Pull-up and pull-down resistors are required in many logic systems to provide a valid logic state when a wire connected to a CMOS input is unused."

    Yes, this is my exact case because the buffer is located at the input. I asked because according to my knowledge, digital circuits in general can have an unexpected output if the input is not well defined (LOW or HIGH). For my application I assumed I need to pull down the input.

    The bottom line is that I want the input to be LOW when the input wire is not connected. Therefore the output would also be at a LOW state. On the other hand, when the input wire has a HIGH value, the output should be HIGH.
  • 0 in reply to Ahmed Abed Benbuk
    TI__Guru* 96947 points
    Hi Ahmed,
    So, the SN74AUP2G17 is connected to the CMOS input of another device - I understand that.

    You are correct that CMOS inputs need to be well defined (ie LOW or HIGH).

    My source of confusion comes from your use of the term 'input' without context -- the system you have described has 2 inputs and 1 output, one input and output on the SN74AUP2G17, and one input on the "digital circuit." (technically the 2G17 has 2 inputs and 2 outputs, I'm only considering 1 channel right now).

    I would recommend connecting a 10kohm resistor directly from the input that is to be disconnected to GND, regardless of which IC the input is on. This will allow the input to be controlled by a relatively weak driver, and it will hold the input at 0V when it is disconnected.
  • 0 in reply to Emrys Maier
    Prodigy 30 points
    Yes exactly, I am using one of the SN74AUP2G17 inputs and the other is not used. I will connect a 10k resistor to the ground, and mark this question solved.

    Thank you.