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SN54AHC123A: Monostable Multivibrator

Ray Vincent Aranzaso
Mastermind 18180 points
Part Number: SN54AHC123A
Other Parts Discussed in Thread: SN74LVC1G08

Hi,

Good Day. I have a customer who is working with SN54AHC123A. Please see below his query for your reference. Thank you very much.

I am looking for support in generating edge triggered pulse widths between 10 and 25 ns. According to the sn54ahc123a datasheet: https://www.ti.com/lit/ds/scls352h/scls352h.pdf it appears that according to pp. 5 a 5 ns output pulse is possible. However an app note I found: https://www.ti.com/lit/an/scla014/scla014.pdf?ts=1649785130686&ref_url=https%253A%252F%252Fwww.google.com%252F says on pp. 3 that the minimum output pulse duration is 290 ns (but this seems in conflict with the datasheet--in particular pp. 11, fig. 5--though that app note is no longer linked on the product website so maybe that information is out of date). Can you achieve pulse lengths as short as 10-25 ns with that part, and if so, how can that be achieved? If not, is there another part I might consider? Thanks,

scls352h.pdf

scla014.pdf

Best Regards,

Ray Vincent

  • 0
    TI__Guru* 97066 points

    Hi Ray,

    You won't be able to get an output of only 10 to 25ns from this device.

    You're looking at two different things.

    Timing Requirements are those inputs provided by you (ie your system) to the inputs of the device. These are limitations in how you use the device, and are not related to the measured output characteristics. The highlighted spec above says that you must put in a pulse that is at least 5ns long to trigger the device.

    Switching Characteristics are the measured output capabilities of the device. The highlighted spec above says that the device will have up to 240 ns output pulse width across process and temperature variations, with a typical value of 167 ns with the given values of Cext = 28pF, Rext = 2 kohm, CL = 50pF.

  • 0 in reply to Emrys Maier
    TI__Mastermind 18180 points

    Hi Emrys,

    Good Day. Please see below the response of our customer to your reply. Thank you very much.

    What is the minimum pulse width achievable with this part? If this part cannot achieve a 10-25 ns part, is there an alternate part that would?

    Best Regards,

    Ray Vincent

  • 0 in reply to Ray Vincent Aranzaso
    TI__Guru* 97066 points

    Hi Ray,

    I would recommend to use the device as described in the datasheet.

    -

    As for getting a 10 to 25ns pulse, assuming a rising edge trigger and an input pulse width longer than 25ns, I would recommend using this circuit:

    The RC will produce a delay of approximately 17.5ns (to Vcc/2, or 0.7 tau). With a typical AND gate such as the SN74LVC1G08, the circuit should produce an output pulse of approximately 17.5ns, however it will vary (thus why I chose a value right in the middle of your desired range).

    Changes in the RC values as well as the input thresholds of the device can cause significant variations in the pulse width.

    If you need more precision than this, I would recommend using a crystal oscillator circuit for timing -- but the circuit would be much more complex than what is shown here.