I have a circuit containing SN54LS123 (Datasheet) IC. I want to know the output pulse width but unable to figure out the value of K.

The formula is PW = K * R * C.

The values of K is given in the plot (how to read figure) also there are two formula (page no.8 for LS123) one for capacitor >1uF and second for capacitor < 1000pF. I am confused. I want to know the exact value of K for

  1. R = 75k and C = 0.022uF
  2. R = 24K and C = 200pF

enter image description here


I found a document "Designing With the SN54/74LS123". This document discuss the Pulse Width calculation but one strange thing is that it contains two similar equations (comparing to datasheet) with different capacitor value ranges. One for >1uF and second for < 1uF. But again the curve in the plot for K values.

  • \$\begingroup\$ You may have looked at some figure or seen some instructions but you gave no indication where you found it or exactly what you saw. You will have to provide more information in order for there to be any real help available. Maybe a start would be a link to a data sheet for your part. \$\endgroup\$ Apr 15, 2020 at 11:52
  • \$\begingroup\$ @MichaelKaras updated question. Thank you \$\endgroup\$
    – user777304
    Apr 15, 2020 at 11:57
  • \$\begingroup\$ Why do you need to know the exact value of K if you can't know the exact value of R and C? The value of K is probably dependent on internal circuit parameters and subject to significant variation. \$\endgroup\$ Apr 15, 2020 at 12:01
  • \$\begingroup\$ According to your figure the value of K is independent of the value of R. So for any given C there is a nominal value of K as can be read from the chart. I say nominal because as @ElliotAlderson said there will be variation from part to part, from temperature to temperature and from VCC to VCC. It may be possible to derive a curve fit for the curve in the chart that could give you an equation to compute nominal K from the value of C but that is something you have to do on your own. The curve in the chart is almost for sure derived from actual part test data accumulated by the manufacturer. \$\endgroup\$ Apr 15, 2020 at 12:12
  • \$\begingroup\$ My target is to write a Verilog code equal to the given circuit which uses SN54LS123. I only have schematic, I do not have actual hardware to check the actual output pulse width. This is why I need to get the approximate K value to generate the pulse width. I think as @MichaelKaras mentioned I have to generate a curve equal to the given chart and get value of K from there. \$\endgroup\$
    – user777304
    Apr 15, 2020 at 12:33

1 Answer 1


For very small C, the device/node parasitics, plus the internal transistor delays in switching and detecting, alter the K

For very large C, there will be self-heating as the internal discharge-the-cap transistor may even exit the desired "saturation mode" and greatly extend the discharge time.


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