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SN74LVC1G123: configure as noretriggerable monostable?

Adam Daluga
Expert 6385 points
Part Number: SN74LVC1G123
Other Parts Discussed in Thread: SN74LV221A,

Hello,

Can the part be configured as non-retriggerable monostable?

I plan to use it for an output pulse between 1 to 3 seconds whenever input is triggered, like using 1.5Mohms and 1uf for ~1.5secs. Can I use larger capacitance?


Thanks,

Adam

  • 0
    TI__Guru* 97046 points

    Hi Adam,

    The SN74LVC1G123 is retriggerable only. If you'd like to use a non-retriggerable device, the SN74LV221A is a good option.

    Large R and C values aren't a problem for most of our monostable multivibrators. The main concern will be the leakage current into the Rext/Cext pin. I would recommend to avoid having more than 0.5V across the timing resistor to avoid issues (less voltage is better for timing accuracy).

    In the SN74LVC1G123, for example, the Rext/Cext leakage value is 0.25uA max, so the largest resistor I would recommend is 0.5V / 0.25uA = 2 MΩ. The SN74LV221A has a larger leakage current of 2.5uA, so I wouldn't exceed 200 kΩ with that device. Note that these are _max_ leakage values, so they are rated only under the worst conditions (usually max temperature).

    The capacitor can be extremely large on these - some datasheets even list the value as "unrestricted" -- the SN74LV221A is one of those. To get about 2 seconds of output pulse, I would start with a 220uF capacitor and 10kohm resistor, and then adjust the resistor to get the precise value you want after building the prototype.