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SN74LVC1G123: 74LVC1G123 Large Pulse duration

SYLVAIN BOURIOT
Intellectual 440 points
Part Number: SN74LVC1G123

Hello,

I need to generate an output pulse with a width between 1.2 and 1.5 seconds.

Looking at the SLA720a datasheet and application note, the maximum duration is obtained with a 200k resistor and a 1µF capacitor.

Can I increase the value of the resistor and capacitor to obtain a longer pulse duration?

I saw in the application note that as the capacitor increases, the K constant decreases. . Is possible to generate a 1.5 s pulse with this IC?

Best Regards.

Sylvain

  • 0
    TI__Genius 9401 points

    Hi Sylvain,

    Yes, the SN74LVC1G123 can generate a 1.5 second pulse. The capacitor and resistor size will need to be determined via prototyping. I'd recommend limiting the resistor to a maximum of 500 kOhms. If you tell me more about your application, especially the Vcc, I can provide some prototyping tips.

    Best,

    Ian

  • 0 in reply to Ian Graham
    Intellectual 440 points

    Hello, thank you for your reply.

    I don't need a precise duration, but I must guarantee a minimum pulse duration.

    That's why I need to estimate the variation of this pulse duration on a batch of components to correctly size R and C.
    The VCC is 3.3V.

    Best Regards.

  • +1 in reply to SYLVAIN BOURIOT
    Guru 316690 points

    The SN74LVC1G123 works by charging the capacitor through the resistor, and triggering when the voltage reaches a certain value. (See Designing With the SN74LVC1G123 Monostable Multivibrator.)

    The charging current can be reduced by the leakage current of the SN74LVC1G123 itself (which is very small), and by the leakage current of the capacitor. Large electrolytic capacitors can have quite high leakage currents; this is why it is recommended for the resistor to be small enough so that the charging current is much larger than the leakage current.

    So your estimate is affected mainly by the characteristics of the capacitor.