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Question about CMOS outputs

B

Bill Bowden

Jan 1, 1970
0
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
 
J

John O'Flaherty

Jan 1, 1970
0
Bill said:
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.

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.
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?

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).
 
Bill said:
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

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.
 
B

Bill Bowden

Jan 1, 1970
0
John said:
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).

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
 
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