# Logic level for system lower than IC's recommended logic level

My system has a logic-level and logic-supply-voltage of 2.4V. I want to use a particular IC in it (which basically operates as a shift register), and the datasheet says the minimum recommended logic supply voltage is 3V (see image below). Would the IC still operate and communicate in the circuit... in other words: could I get away with it? And if so, what problems might i encounter or be more likely to encounter?

Cheers.

• A logic supply level is not the same as the voltage that will be IDed as a HIGH. You're looking a tv the wrong data sheet entry – Scott Seidman Mar 20 '18 at 11:17
• Vih = 0.8 *Vsupply – Scott Seidman Mar 20 '18 at 11:20
• 2.4V is unusual. What circuit outputs this? – bobflux Mar 20 '18 at 11:28
• The supply voltage for the MAX6921 would also be 2.4V, NOT just the logic interface. I've edited the post to reflect this. And I'm just trying to cut out adding another voltage regulator in the circuit by using the same regulator for both the logic ICs and a VFD cathode. – Joe Mar 20 '18 at 12:57
• The "don't do this" is even more clear. I don't know the failure modes if you don't power the chip correctly, but AT THE VERY LEAST you need to power the chip correctly. – Scott Seidman Mar 20 '18 at 13:06

Once you enter the world of operating outside IC specifications, you are inviting in shaky, unstable behaviour and odd faults that take time to track down.

You may see a cost saving in using a chip outside its specification. That saving may seem attractive or worthwhile because the proper chip is much more expensive, the board would need rework to use the proper chip, the chip would need extra interface components etc. That cost may be multiplied by the number of 'wrong' boards you have made and in stock.

That must be traded off against the cost of testing and proving sufficient boards to be sure it will work and the cost of tracking down obscure faults that might result from it. And most of all, the possible costs of recalling and replacing this dodgy design with the corrected boards that you avoided making in the first place and that have to be made after all.

Its worth noting that reliability faults or errors through instabilities will not reveal themselves until you have a large-enough number of units in use (read up on MTBFs). That may mean one unit before the fault shows, it may mean a hundred or a thousand - it depends upon the components and the design.

So the reason why going outside IC spec' is a bad idea may not become apparent until you have made and used lots, when the corrective action costs have got higher. So just running a few in a lab' under test for a bit cannot to be taken as definitive proof of reliability.

As the going-wrong costs usually considerably outweigh the cost savings if taking a chance goes right, then I would never use ICs outside of their rated specifications.

You know your circumstances but I recommend that you do not. Select the correct IC instead or add a voltage translation circuit to get the right voltages in/out of that IC.

You are looking at the wrong area of the data sheet. To know if your chips can talk to this chip, you need to look at the logic levels:

Here, you see that $V_{IH}$ is minimally $0.8 \times V_{CC}$. If you power it off of three volts, this is 2.4V. To me, this is a little bit close to the edge, and I would consider a logic converter of some sort.

The risk is pretty obvious -- the IC will misinterpret what your circuit is trying to tell it, and some HIGHs may well be read as LOWs.

Good job noticing that this is a likely problem before you threw your eggs into this particular basket, but it is usually very wrong to "take the risk". Data sheets exist for a reason.

• I replied to your comment on my post (above), and edited the post to clarify what I was asking. The 2.4v isnt just the logic-level for communication. It is also the supply voltage I intend to use. – Joe Mar 20 '18 at 13:04