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Original question:

SN74LVC1G123: SN74LVC1G123 minimum retrigger time calculation

SN74LVC1G123: Monostable multivibrator retriggering

Chetan Karkera
Prodigy 90 points
Part Number: SN74LVC1G123

Tool/software:

Hello 

I have recently tested the SN74LVC1G123 part to confirm the retriggering time as mentioned in the datasheet, 

For 300mHz to 100Mhz with Rext=325K and Cext=10uF , LED was glowing continuously with output around 3V (with 250mVpp ripples at the higher frequencies).

So i believe retriggering happening all in the range of 3.33s to 10ns pulse period. 

But  datasheet says, the minimum retriggering requirements, for example if we take 0.01uF as Cext then for 3.3V the minimum retriggering time should be 1us or above? 

If so may i know how the retriggering is happening for above test circuit for such a wide range of frequencies, am i missing anything here?

Note: Sorry for the explanation without pictures, I cant share the screen shot of results due to company policies 

  • +1
    Guru 295220 points

    The retriggering time is measured for only two input pulses. If you are below the minimum retriggering time, the second trigger will not be recognized, and the output pulse length will not be extended.

    But as far as I understand, your input signal into the trigger input is a square wave. When the second input pulse is not recognized, the third, or a much later one, might still extend the output pulse.

    Your input signal should be two pulses, with the desired time interval to check, and then a pause that is longer than the output pulse length.