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CD4541B-MIL: shorter count value than expected

Mathias Sellini
Prodigy 30 points
Part Number: CD4541B-MIL

Hello,

I am actually designing a CD4541 to allow it to count at 9.5 s approximativly

I have done several tests with Rtc and Ctc couples but none of them gave me a correct value. Each test i've made gave me a time shorter than expected.

For example, i have settle a count value of 8192, a Rtc of 105 kOhm and a Ctc of 4.7nF. I am expecting a time value between 9.11s and 9.48 s but it gave me 8.76s on the five samples i've built.

And the same thing happened with each couple i used.

Is the formula in the datasheet correct ? the 2.3 factor is the most precise one or has it been shortened to be more "readable" ?

Please let me know .

Best regards

  • 0
    TI__Mastermind 27045 points

    Hey Mathias,

    Our logic expert is out at the moment and will be able to get back to you next week.

    Thanks,

    Daniel

  • 0 in reply to Daniel Hartung
    TI__Guru 62930 points
    Daniel,
    There is quite a bit of variability due to tolerances of capacitor and resistor.
    I do not have any specific data indicating that the scaling value 2.3 is incorrect. Most variability will be due to the tolerances of the resistor and capacitor. Based on your range reported, you are using 1% tolerance for both?

    Additional variability can come from device process and voltage. The switching threshold of the input will be function of voltage and is near linear with VCC.

    The 2.3 value is a nominal value and will have some variability. Unfortunately, this range is not a specified parameter.

    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    Prodigy 30 points

    Hello,

    here are more informations :

    - Rtc and Ctc have both a 1% tolerance

    - the timer's VCC is 4.7V.

    We tried to move VCC's value from 3.3 to 10V to see if it affect the internal oscillators behave and the result is that it doesn't.

    As you guessed, the threshold was the parameter we were the most afraid of because we have no other indications than a nominal value.

    Is it possible for you to have more detailed information on it ?  

  • 0 in reply to Mathias Sellini
    TI__Guru 62930 points
    Unfortunately, there is very little additional documentation. This device was acquired by TI from Harris around 2003.
    There is a note in the datasheet indicating that you can drive pin 3 directly with clock if more accuracy is required.

    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    Prodigy 30 points

    Ok we will deal with it then

    Thank you for your quick answers