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LMC555

Mike Edwards
Prodigy 30 points
Other Parts Discussed in Thread: LMC555

LMC555 Timer

We have a costumer that is concerned that the data sheet does not explicitly state the minimum operating frequency. Their desire is to operate at 1 HZ. I know this is well within the realm of functionality from experience but this is not clearly stated in the data sheet.

On page 4 of the data sheet, the astable frequency is stated under one test condition and the maximum frequency is also stated, however even in the case of maximum frequency it’s just stated as a typical and therefore of little value, furthermore there is no statement of minimum frequency, as seen below:

         

Their view is that if it is not specified in the electrical characteristics then it may or may not work at any frequency beyond the minimum and maximum listed.

There is a chart Figure (10) and equations (4) which shows operation at 1HZ. But there is no guarantee that part will work at this speed. How can we justify / guarantee operation if the data sheet does not explicitly state the minimum operating frequency?

  • 0
    TI__Genius 10320 points

    Hi Mike,

    I've contacted the appropriate product engineer for this device to answer your question.

    Gabe

  • 0
    TI__Guru 52356 points

    Hello Mike,

    There is no guaranteed minimum because there is no lower frequency limit for the LMC555. There is nothing inside the LMC555 that would limit the lower frequency. The external components are the limiting factor. The frequency is set by the external R/C combination.

    The major limitation of the lowest frequency is the capacitor leakage and soakage - which should not be a major issue at 1Hz (1uHz maybe).

    The formula is the guarantee (at frequencies below 10KHz). As long as the THRESH input levels (eventually) reach the required 1/3 and 2/3 thresholds, the output will respond. The signal path is DC coupled, so it does not care how "slow" the input is - as long as the capacitor charge eventually crosses those thresholds. Many people use this device for time delays of hours - so 1Hz is easy.

    The LMC555 will happily oscillate at 100Hz, or 10Hz, or 1Hz, or even 1mHz. Just scale the R/C appropriately. Use a good film type for lowest leakage.

    Regards,

  • 0 in reply to Gabe Ayala
    TI__Prodigy 30 points

    This is exactly as I thought! I’ll use your explanation with the costumer, I am sure they will understand.