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SN74LVC1G123: maximum resistance and capacitance on output pulse duration(tw)

Shinichi Inoue
Guru 21721 points
Part Number: SN74LVC1G123

Dear Specialists,

My customer is using SN74LVC123 and has a question.

I would be grateful if you could advise.

---

In order to make the pulse width 1 s, Rext = 1 MΩ and Cext = 1 μF.

However, there are things that do not work properly（1/500）.

(1) Is there a problem with the resistance (1MΩ)?
If there is a problem, please tell me the reason and the maximum value(Ω).

(2) Is there a problem with the capacitor value?
If there is a problem, could you please let me know why and the maximum value(μF)?

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Genius 16735 points
    Hello Shinichi,

    When you say things do not work properly, what do you mean?
    Is the pulse width drastically different from 1s?
    Are the resistor and capacitor accurate?
    Are you doing a temperature swing or stress tests?

    The datasheet does not mention an issue with using such large resistor and capacitor values so I don't believe they are the problem.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Guru 21721 points
    Hi Karan,

    Thank you for your reply.

    The customer says when CLR is L to H, the pulse wasn't output.

    At first, I think 1Mohm may be too big, I consulted to you.

    You mention It's no problem, I consider the other reason.

    I appreciate your great help.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    TI__Genius 16735 points

    Hello Shinichi,

    After further considering I realize you are correct, I believe that your resistor value of 1MOhm is actually too large. The resistor value is probably causing a voltage drop from Vcc that is too large for the switching to occur. Like you mentioned no pulse output. This happens because the capacitor doesn't get to charge up to a high enough voltage fast enough considering this large resistor drop. If you reduce the resistor value and increase the capacitance it will improve your design.

    Thanks!
    -Karan

  • 0 in reply to Karan Kotadia
    Guru 21721 points

    Hi Karan,

    Thank you for your reply.

    I understand too large resistor value is one of the reason.

    I'll share this information with the customer.

    If he has an additional question, I consult you again.

    I appreciate your great help.

    Best regards,

    Shinichi