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SN74AHC123A: K Value Over Cap and Temp

Jacob Olstad
Intellectual 1270 points
Part Number: SN74AHC123A
Other Parts Discussed in Thread: SN74LVC1G123

Hi,

Looking through the datasheet for the SN74AHC123A figure 9 shows the K value over capactiance and states that values of .001uF or greater equales a K value of 1 but the graph looks more like .001uF is equal to a K factor of 1.1 and figure 8 shows a C value of .1uF varying all the way down to .95.

On top of that I don't see any data on how K varies with temp.   Figure 6 shows timing changes over temp but no K value.

Ultimately trying to fill in the below table:

Capacitance

779pF

861pF

Temperature

-40C

+85C

-40C

+85C

K min

 

 

 

 

K max

 

 

 

 

Thanks,

Jake

  • 0
    TI__Guru* 96842 points

    Hi Jake,

    There's a great app note on this part located here: 

    It looks like you're trying to determine the output pulse width with a great deal of precision.  I should warn you that monostable multivibrators are notoriously not precise -- primarily because of the external components being used.  While our device might only shift the output pulse width by 1 or 2% over temperature, the external capacitor's value can shift by more than 100%... and that's what directly controls the pulse width.  Note that a capacitor's manufacturing tolerance (for example, a 5% capacitor) is not its temperature variation -- sometimes that value is much harder to find.  It's not uncommon to see numbers ranging from -50% to +125% or worse.

    Can you tell me about your application?  What are you trying to achieve?

  • 0 in reply to Emrys Maier
    TI__Intellectual 1270 points

    Hi Emrys,

    Thanks for the quick response.  The capacitor I can calculate for and use COG caps with fairly minimal temp variation.  Same goes for the external resistor.  However, it's still unclear to me the correct K value and how it varies with temperature.  I have a fair amount of slop to play with in the end but I need to make sure that I'm within some limit for the final design.

    Do we have data on temp variation for K ?

    Thanks!

    Jake

  • 0 in reply to Jacob Olstad
    TI__Guru* 96842 points
    It appears that I entirely forgot to mention that Figure 7 in the app note I linked shows variation in pulse width over temperature. I'm afraid that I don't know the particular test conditions - the change could be due to the external components or the device itself.

    When I have tested this in the past with similar devices (SN74LVC1G123) I found a maximum of ~2% error introduced from the IC itself, and the remainder was caused by the external components. Figures 7, 8, and 9 in this app report show my results: www.ti.com/.../slva720.pdf

    As a more direct answer to your question - no, I do not have char data for K over temperature for the SN74AHC123A. Personally, when I design circuits with one-shots, I give quite a bit of margin (~100%), and I always prototype with the intent of fine-tuning the pulse width.