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SN74AXC4T774: SN74AXC4T774 - output with pull up resistors.

Sunset
Genius 13985 points
Part Number: SN74AXC4T774

Hello Team,

the datasheet for the SN74AXC4T774, as other devices without the schmitt trigger input, points out that the input should never float.

There is also the application note describing the reason. This is ok and understood.

Not being floating, brings also a minimum rise time for any voltage change. To avoid being in the "unknown" state area for long period.

  • If I have only 10pF load, what could be the pull up resistors I can use to avoid having floating input and not create problem for the rise/fall time ? (datasheet has Input transition rise or fall rate being max 5ns/V
  • Is it the pull up value a realistic pull-up?

Thanks,

SunSet

  • 0
    TI__Mastermind 18010 points

    Hi Sunset,

    With a 10pF load and an input transition rise and fall time of 10nS/V, the pull up resistors should be 333.33Ω. The input transition rise and fall time is 10nS/V as seen in the recommended operating conditions of the datasheet. Here is our FAQ "How do I size pull-up or pull-down resistors?" for further reference.

    Regards,

    Sebastian

  • 0 in reply to Sebastian Muriel
    TI__Genius 13985 points

    Hello Sebastian,

    your answer brings me to the next bullet.

    • Is it the pull up value a realistic pull-up?

    333 ohm with 3.3V on the second driver, we get 40mA for pull-up resistors for total current drive of 100mA.

    datasheet allows max 5ns/V, which would lead an even smaller resistor.

    This actually lead me of not using the pull up resistor for the SN74AXC4T774.

    In the link you have pointed out, of using "10K and test it", you are just out of the 5ns/V.

    10K cannot be used.

    Sunset

  • +1 in reply to Sunset
    TI__Mastermind 18010 points

    Hi Sunset,

    The AVC4T has an input transition rate of 5ns/V but the AXC has a 10ns/V input transition rate. 

    When configuring the IO as an input it will not sink any current because it is a Hi-Z input. Using the RC Time constant equation (t = 3*R*C)  in the FAQ, we find the total time by multiplying the input transition rate 10ns/V by the driver voltage 3.3 V. 10ns/V * 3.3 V = 33 ns total time. Now by plugging in our total time, R, and C values into the RC time constant equation we get 1kΩ for the pull up resistor. 

    Regards,

    Sebastian 

  • 0 in reply to Sebastian Muriel
    TI__Genius 13985 points

    Thanks,

    my typo on the device, I always meant the 5ns/V since we are using the AVC4T  and not the AXC.

    Sunset