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CD14538B

James McKnight
Prodigy 40 points
Other Parts Discussed in Thread: CD14538B

I am trying to replace a 74C221, as it is obsolete, with the CD14538B.  The data sheet states wide range of output pulse widths.  What is this range? I would like one of the circuits to have a 2.2uS period and the other to have a 30uS period. For the 30uS period the equation T= 0.5 RC gives R = 5K Ohms and C = 12nF.  Because C is greater than 5nF do I use the equation T = RC.  This yields C = 12nF and R = 2500 ohms which is below the 4K minimum.  So if I half C to 6nF this equation yields C = 6nF and R = 5K Ohms.  So should I use C = 6nF and R = 5K Ohms to achieve my 30uS pulse?  As for the 2.2uS period the equation T = 0.5RC gives R = 4K and C = 1.1nF this falls in the acceptable values for C to use this equation.  So should I use R = 4K and C = 1.1nF for my 2.2uS pulse?  I had other questions about the part but DigiKey was able to provide those answers.

  • 0
    TI__Mastermind 46480 points

    It is best to stay withing the recomended operating range. this will keep the formula more accurate.

    What is this range?  Not sure. You should have no problem getting the 2 widths you need.

     For the 30uS period the equation T= 0.5 RC gives R = 5K Ohms and C = 12nF.  Because C is greater than 5nF do I use the equation T = RC.  Yes

    This yields C = 12nF and R = 2500 ohms which is below the 4K minimum.  This will work but the formula may not be accurate here.

    So if I half C to 6nF this equation yields C = 6nF and R = 5K Ohms.  So should I use C = 6nF and R = 5K Ohms to achieve my 30uS pulse?  This would make the formula more accurate.

    The formulas are most accurate  when you use the C values shown in the datasheet .  Using different values just makes the formulas less accurate. It daes not mean the part will not work in that range. Also keep in mind the formulas will not be exact. they will get you close and then you will tweak from there.

  • 0 in reply to Chris Cockrill
    Prodigy 40 points

    When you say values shown in the data sheet are you referring the bottom of page 1 where it says the minimum resistance is 4K and min and max for capacitance are 0pF and 100uF, respectively.  If so I am within the range.  If not where would I find these values?

  • 0 in reply to James McKnight
    TI__Mastermind 46480 points

    Yes

    You are within spec.