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Question about CMOS outputs

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 load.

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?

-Bill

Reply to
Bill Bowden
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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).

-- john

Reply to
John O'Flaherty

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.

OR

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.

Reply to
JedOs86

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.

Thanks,

-Bill

Reply to
Bill Bowden

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