Tying unused pins on CMOS inverter

Thread Starter

NomadAU

Joined Jul 21, 2012
37
Hi,

I've got a circuit that uses 2 of the 6 inverters on a CMOS 4069 chip. I had read that it was a bad idea to leave unused pins floating, so without doing much research, I decided to tie all of the unused input and output pins to ground....and this doesn't seem to have been a very good idea. I did notice that I was pulling around double the expected current through the circuit with this arrangement...and then found that the IC had been totalled :mad:

So, question is, which pins should I tie? Now that I've given it a bit more thought, I am thinking of just grounding all of the unused input pins, and leaving the outputs floating.

Is this a reasonable solution?
 

tshuck

Joined Oct 18, 2012
3,535
Hi,

I've got a circuit that uses 2 of the 6 inverters on a CMOS 4069 chip. I had read that it was a bad idea to leave unused pins floating, so without doing much research, I decided to tie all of the unused input and output pins to ground....and this doesn't seem to have been a very good idea. I did notice that I was pulling around double the expected current through the circuit with this arrangement...and then found that the IC had been totalled :mad:

So, question is, which pins should I tie? Now that I've given it a bit more thought, I am thinking of just grounding all of the unused input pins, and leaving the outputs floating.

Is this a reasonable solution?
Never tie the outputs to ground! Simply don't connect your outputs if you don't need them, but tie your inputs to ground...
 

crutschow

Joined Mar 14, 2008
24,402
If you think about it, you are shorting the outputs to ground. So if the inputs are also shorted to ground, the the outputs are trying to go high but can't. So they are all outputting their maximum current into a short. Makes for excessive supply current and enough power dissipation to zap the chip.
 

wmodavis

Joined Oct 23, 2010
739
It is the correct solution to tie only the inputs to common. There are many ways to learn things. You picked one that will likely fix what you learned in your mind permanently.
 
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