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LMC555 Time Delay

alpha
Expert 2530 points
Other Parts Discussed in Thread: LMC555

Dear specialist

Our customer has been considering LMC555.

They would like to configure that Time delay will be more than 100s.

Question1
Is it possible to configure more than 100s delay ?

Question2
If it is possble as above,is time delay configured 1.1×R×C?

Best regards.

  • 0
    TI__Mastermind 24205 points

    Hi Alpha,

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

    Regards

    Arvind

  • 0
    TI__Guru 60476 points

    Hello Alpha,

    1. Yes.

    2. Yes.

    You will need to use a low leakage capacitor. Avoid using electrolytic or tantalum caps. Low leakage poly types are recommended (although they are physically larger). Ceramic types are okay..but they can have problems with piezoelectric and/or microphonic effects at low currents.

    Large electrolytic capacitors (>100uF) may have leakages that will swamp the charging current. So you will want to use large value resistors (Megohms) and keep the capacitor value as low as possible. High value, low leakage poly caps are large and expensive, and difficult to find in values over 10uF.

    The LMC555 has CMOS inputs, so the timers input currents are in the pico-amps, so you do not have to worry much about the bias currents, but you will have to be aware of external layout leakages. Keep the timing resistor and cap node neat and compact.

    I would start with a 4.7uF cap and 20Meg resistor. The 4.7uF cap can be made of several paralleled smaller (1uF) capacitors if you cannot find a small sized 4.7uF.

    BTW: If you do use a rolled poly cap, tie the striped end to GND. The stripe is the outer foil side and will shield the inner electrode.

    Regards,