SN74AHCT123A and SN74LS221 - some questions

Hello Hirotaka Matsumoto,

For SN74AHC123A:

On page 4 of the datasheet, the minimum pulse duration is listed as 5ns.

The output pulse duration can be calculated from this equation:
tw = K x Rext x Cext,

K is a multiplier that depends on the capacitor value. Page 7 of the datasheet explains that K is 1 for any Ct value greater than or equal to 1000pF. For a capacitor value less than 1000pF, I would refer you to figure 9 on page 10 of the datasheet to help gauge the K value. Also, figure 5 of the datasheet shows variation in tw based on varying Rt and Ct values.

For SN74LS221:

The output pulse duration can be calculated from this equation:

tw = Cext x Rext x ln(2) ~= 0.7 x Cext x Rext.

The minimum Rext and Cext values are listed on page 6 of the datasheet. So using those and the equation, the pulse width can be calculated. Figure 6 on page 13. This plot shows output pulse based on resistor and capacitor values. Based off this plot, a 30ns output pulse is achievable using a 4kOhm resistor with a 10pF capacitor.

To answer your second question, if Cext is left open (Cext = 0) a typical output pulse duration of 30ns can be achieved with a 2kOhm resistor, based off page 2 of the datasheet.

Hope this answers your questions!

Regards,
Joe Stephan