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Calculation method of SN74LVC1G123

Satoshi
Guru 19645 points
Other Parts Discussed in Thread: SN74LVC1G123

Please let me know about calculation of SN74LVC1G123.

-------------------------------

①Equation of Pulse retrigger time(trr)

②About Pulse width(tw), calculation method of "K"

   What meaning is "K"? (Calibration?)

   Not only the graph,  I want to know accurate value.

※Vcc:3.3V, 330kΩ, 1μF

-------------------------------

I found attached application note, but not understand perfectly.

slva720.pdf

Best regards,

Satoshi

  • 0
    TI__Guru 62170 points
    Satoshi san ,

    K is a constant factor , and depends on the cap used as per the graph shown . Values are estimate but for reasonable approx. for cap values not on the graph , you can use 0.9 .
  • 0
    TI__Guru* 97046 points

    Hi Satoshi-san,

    I think the best way to explain the K value is to say that it is a correction value.

    In an ideal monostable multivibrator, the output pulse would always be exactly Rext * Cext in length.  In a real part, however, the output pulse length is dependent on a number of factors, which are all encompassed by the constant 'K'.  There isn't really an equation to find K (at least that I know of), I have found it through experimentation.

    I always recommend to set the part up with the components you plan to use (not just the values, but the specific components), and then test the output pulse length over expected temperature and Vcc.  Sometimes value adjustments should be made to maintain the desired pulse length.

    As for the retrigger time, this has to do with the discharge rate of the timing circuit.  although it is not precise, I would recommend using 0.1*Rext*Cext for this value to be safe.  The largest value I have seen in our data is actually 0.09*Rext*Cext.  The multiplier decreases inverse to capacitor size -- ie a 1uF capacitor has a multiplier closer to 0.00003, and a 100pF capacitor is closer to 0.09. 

  • 0 in reply to Emrys Maier
    TI__Expert 3435 points
    does your recommendation (0,1 * Rext * Cext) refer to my actual system (121k / 100nF) which is to be used on our board?
    Is your recommendation based on any technical documentation (... I have seen in our data ...)?
    My superiors need to have an official source for any technical suggestion.