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SN74AHC123A: tw > 20ms

Roy Chou1
Genius 14775 points
Part Number: SN74AHC123A

Hi Team,

I want to check spec with you. I find there is no max value showing on datasheet. If our tw need to > 20ms which means that we don't need to connect Rext/Cext, is it correct? Thanks.


Regards,

Roy

  • 0
    TI__Mastermind 24915 points

    Hello Roy,

    Are you saying that you want the output Q pulse width to be greater than 20ms?

    You must have an Rext and Cext connected and all pins configured appropriately and then follow figure 5 to get the appropriate/desired timing. It will take somewhat of a large RT and CT to get that large of a pulse., and the pulse will likely vary in its precision quite a bit, but if you just need a pulse longer than 20ms than getting an exact number is not that important.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    TI__Genius 14775 points

    Hi Michael,

    Thanks for info. I found below spec. The spec shows the max time duration. Can you let me know the tw(max) is related to Rext and Cext?

    If we want to tw > 20ms, I need to set Rt = 200k and Ct at least larger than 10^5 pF. Is it correct?

    Roy

  • 0 in reply to Roy Chou1
    TI__Guru* 96937 points

    Hi Roy,

    The output pulse width is tw = K * R * C

    The maximum output pulse width you highlighted was measured across PVT and is not something you can calculate. It is only given for the values you highlighted.

    As C gets larger, K generally gets closer to 1, however with this device it appears that it approaches closer to 0.9.

    So, for example, if you were to set C = 1uF, then

    tw = 20ms = K * R * C = 0.9 * R * 1uF

    R = 20ms / (0.9 * 1uF) = 22.2 kohm

    A larger value of resistor will provide a larger delay, so to ensure tw > 20ms, I would use a 33 kohm resistor.

  • 0 in reply to Emrys Maier
    TI__Genius 14775 points

    Hi Emrys,

    Thanks for information. Is it possible that set Tw larger than 1s if we set appropriate R/C value?

    Roy

  • 0 in reply to Roy Chou1
    TI__Guru* 96937 points

    Hi Roy,

    Yes - there is no limit on the C value. I would recommend keeping R less than 500kohm though.