• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 18 subscribers
  • Views 395 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

CD14538B

John Michael75660
Prodigy 30 points

I am using a CD14538BF3A multivibrator with a 180ohm current limiting resistor. Adding this resistor was causing significant variation in output pulse width at room temperature, and further deviations over temperature.

Why does adding a current limiting resistor reduce the pulsewidth of the circuit, and why does this cause increased variation over temperature?

Thanks,

  • 0
    Prodigy 30 points

    To try and clarify my question:

    In case of rapid powerdown a 51 ohm resisitor is reccomended to limit current into the IC.  The data sheet also states that this resistor will cause a slight decrease in output pulse width.

    Why does adding this resistor reduce the pulse width?

    What parameters, such as loading or currents, are related  to this effect?

    Are these parameters sensitive to temperature?

    Thanks,

  • 0 in reply to John Michael75660
    TI__Mastermind 46480 points

     

    Why does adding this resistor reduce the pulse width?   You are changing the timing resistor value when you add the 50 ohms.  It should be a very small change and the timing resistor can be adjusted to compensate.

    What parameters, such as loading or currents, are related  to this effect?  Should not affect the load at all

    Are these parameters sensitive to temperature?  Yes,  The resistors and the device will changes slightly with temp. see figure 11 in the datasheet

  • 0 in reply to Chris Cockrill
    Prodigy 30 points

    Thank you for the response,

    If I am changing the timing resistor value by adding the 50 ohm resistor, isn't that an increase in the timing resistor value? If so then using T = Rx * Cx should also increase and not decrease? Why is the opposite true?

    (Rx) * Cx < (Rx + 50) * Cx ?

  • 0 in reply to John Michael75660
    TI__Mastermind 46480 points

    I am not sure. It seems it would also change the discharge time of the capacitor which would also increse the time.  These parts were designed by Harris Semi and the we purchased them. I dont have any appnotes or schematics where we could explain it.