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Question about CMOS outputs

Discussion in 'Electronic Basics' started by Bill Bowden, Aug 12, 2006.

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  1. Bill Bowden

    Bill Bowden Guest

    Looking at the spec sheet for the TLC555 CMOS timer I see the output
    source current is rated at around 10mA. Hooking it up on the bench with
    a 510 ohm load yields about a 4.5 volt swing using a 6 volt supply.The
    positive output is about 1.5 volts less than the supply voltage so it
    apperas to be excessively loaded at 9 milliamps. The output dosn't move
    completely between the rails and loses about 1.5 volts with a 10mA

    Is this normal, and is it safe to operate a CMOS chip with such a large
    voltage drop across the internal parts?

    The power dissipation in this case is only 1.5 times 10mA, or about 15
    milliwatts, so I'm guessing the operation is ok?

  2. Looking at the data sheet (pages 5-7 of the version I saw), I see that
    they describe operation at 3V, 5V, and 15V. The 5V section doesn't
    specify it at 10 mA source current, but at 1 mA. The minimum guaranteed
    output in that condition is 4.1V, or a 0.9V drop. With 10 mA, I think
    it could easily reach 1.5V drop. The 15V operation does specify the 10
    mA source, and has a minimum output of 12.5V, or 2.5V drop.
    Since their data sheet shows 2.5V possible drop at 10 mA, it should be
    alright. They show maximum dissipations in the 1/2 to 1 W range at 25C
    ambient (page 4).
  3. Guest

    The TLC555 is used for extreme low power, as you can see. I suggest
    using the NE556 instead, it outputs many more mA. And its circuiting is
    exactly the same.


    Youre not suppose to directly drive a load from the TLC555, so you
    could try attaching a transistor the output of the TLC555 with a small
    base resistor and a pull-down resistor to GND.
    However, I hooked up CMOS TLC555 directly to a 9V battery and then
    directly attached a standard LED to its output. The chip never heated
    up and still works. Maybe you just have a dud.
  4. Bill Bowden

    Bill Bowden Guest

    Yes, I also have a LM555CN bipolar version that does 50mA source wth no
    trouble, so I'll use that instead. But I have seen references to the
    TLC CMOS version that claim the output moves rail to rail, unlike the
    bipolar version. Turns out, that is only true if the load is very small
    at just a couple milliamps. The bipolar version is much better if you
    don't mind the extra idle current.


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