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SN74LVC1G123: Pin Cext - t ground or not to ground ???

Jeffrey Casey
Prodigy 60 points
Part Number: SN74LVC1G123

Support Path: /Other topics/Design techniques and how-to guides/

I want to migrate a design from the venerable MM74HC4538N to the tiny (and low voltage capable) SN74LVC1G123 one-shot.

My question concerns grounding the "Cext" pin.  You have inconsistent documentation.

Arguments to ground it:    1)  it is traditional in this type of one-shot              2)  Figure (1) of the spec sheet shows it grounded          3)   Figure (1) of the application note SLVA720 shows it grounded

Arguments not to:          1)  Figure (10) of the spec sheet shows it not grounded             2)  The unnumbered figure on page 7 of the app note SLVA720 shows it not grounded

Furthermore, the Q&A of the above app note, question 5.6, specifically deals with this without answering...basically says to "follow what the data sheet says".     Huh?

Please clean up the documentation a bit and advise which is correct for reliable and robust operation.   I am designing this into the fault protection system for a very expensive klystron modulator, and failure here would be catastrophic.

Thanks!

Jeff Casey  /  Rockfield Research Inc  /  Las Vegas, NV  /  casey@rockfieldresearch.com

  • 0
    Guru 316600 points

    Additionally, the description of pin 6 says "Connects only to the external capacitor".

    Anyway, section 5.6 of the application report tells you how to handle conflicting information:

    In some monostable multivibrators the Cext pin is internally tied to ground, but in some it is not. If the datasheet is unclear and the information is required, use an ohmmeter to check the resistance between the two pins. If the resistance reads less than 1 Ω, it is safe to connect both to ground.

  • 0 in reply to Clemens Ladisch
    Prodigy 60 points
    That is exactly what I am talking about -- don't you consider those two specifications to be mutually inconsistent?