Decoupling a cmos based 555?

[chopnhack]

Do you think that a 7555 requires decoupling the input voltage if it is battery fed? Please leave your reasoning in the reply, thanks!

 

[chopnhack]

I don't think it requires it since it is fed by a stable battery - this of course may depend on the current drawn for the circuit. I am basing this on less than 20mA draw from a c2032 3v lithium battery.

 

[Arouse1973]

This will be interesting John. Can't wait to see what people think!
Adam

 

[BobK]

Decoupling has nothing to do with the stability of the power supply. Decoupling is needed when a chip might change it's current draw rapidly. When this is the case, inductance in the wiring can cause a glitch in the power, whether it is a battery or a regulated or unregulated mains fed supply. Since a '555 can cause a rapid change in current as it's output switches, it *might* need decoupling. Whether it actually does or not need one for reliable operation would depend on the particulars of the design.

Good practice would be to always use decoupling. It can't hurt and it might help.

Bob

 

[chopnhack]

@BobK Thanks! That is a great explanation, I was thinking of the cap as a mini battery - keeping the supply rail steady and thus being battery fed, in my design, would be unecessary. Thanks again for the clarity in your answer and a good catch.

Just a note: from some datasheets, it says that the decoupling cap on the control voltage pin is not necessary. This should not be mistaken for vcc.