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Q&A

Why 3V3 instead of 3V?

+5
−0

Usual values used in doing electronics/power supplies are almost always round numbers:

  • 5V
  • 12V
  • 28V
  • 48V
  • 60V

I understand that in some cases IC's are built to accept the voltage directly coming from the lithium-ion cell in order to simplify the power stage, therefore, having 4.2V input.

But, what is the reason behind havin 3V3 voltage level omnipresent instead of 3V?

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6 comments

24 volts is probably used more often than 28 volts and quite possibly 72 volts is more common than 60 volts. 15 volts is also commonly used. Andy aka‭ 3 months ago

I've been using 72V extensively and found that 60V was more common simply because it's in the "Low Voltage" category. It was battery-powered, though. Still, why 3.3V and not 3V? Most(every?) of the IC's accepting 3.3V could accept 3V so I assumed there is some historic reason behind the 3.3 value. 2kind‭ 3 months ago

We could also add 1.8V and 2.5V to the question. Nick Alexeev‭ 3 months ago

It's a number somebody picked from a whole set of tradeoffs. Note that 3.3 V is just about 2/3 of 5 V. Olin Lathrop‭ 3 months ago

Everyone's right in one way. There is no way to close the question other than deleting it so I will leave it here unanswered. 2kind‭ 3 months ago

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2 answers

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This question popped up in the feed and I got curious. Here is what I could find. Note that I am not an IC engineer so my interpretation of some facts may be off.

The 3.3V level is defined in the JESD8 standard. It was made by JEDEC.

3.3V is a stepping stone on the path of decreasing supply voltages. This came about because of the improvements in chip design and introduction of CMOS, which in turn moved the optimal operating voltage down.

This standard is nice in that it also provides some backward compatibility: if you look at it, its logic levels are compatible with TTL, so this means CMOS chips can work with TTL chips. See the chart below for the logic level comparison (Analog Devices). Note that the standard actually allows operation at the 3V level and calls it the "Extended range".Comparison of logic levels.

Why the number 3.3? This seems to go way back in the 90's to the development of the first ICs. I can find some anecdotal evidence about this level being a consequence of RTL design in the early days of semiconductor technology, but nothing that I can reference.

So in short: we use 3.3V now because it is a standard. The value itself comes from the properties of silicon and semiconductors that the early manufacturers used. Why that is is a question for the history buffs on this site.

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2 comments

Nice answer, +1. However, there were ICs well before the 1990s. You may be surprised to find that even back in the 1960s we had sliced bread, flush toilets, and integrated circuits. Olin Lathrop‭ about 1 month ago

Yep... There's an economic reason or semiconductor reason first and then the standard gets drafted... leroy105‭ about 1 month ago

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Actually I think this is a really good question. It's kind of more historical though I guess (and I don't know the historical answer either, but from building products perspective I think this is probably the reason/s).

75V DC is a cut-off value for safety testing on products (OSHA / UL requirements in North America | LVD in Europe). That's a good reason to not use 75V DC.

I think the real answer is probably pretty multi-disciplinary on the economics of producing batteries.

I have some family friends who own battery factories overseas and know a bunch about LiPos from a producing consumer goods perspective and safety issues but I'm no expert on the chemistry of batteries.

For some reason the chemistry of batteries appears to be really easy to produce in 3V & 5V regions. My guess is that the batteries got commercialized first at 3V / 5V and then the ICs were built in order to use the cheapest and readily available batteries.

(Your dollar store AAA battery is 1.5V -- add in 2, and now you are at 3V. So if I'm building semiconductors do I require my end user to go fabricate a custom 4V battery or design the whole thing around the fact they can go to the dollar store and power up my gear for $0.30 of batteries?).

Whole point of hardware is to sell it.

Same thing on the regulatory side: FCC creates new rules for how you can use RF airways, and viola your local RF semiconductor company creates a new chip to maximize usage of the new rules. First comes the regulation and next comes the hardware.

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3 comments

Li/Ion and LiPo are generally around 3.8 - 4V somewhere though, at least modern ones. And I think 3.3V goes back to the early 1990s somewhere, it probably pre-dates such batteries. So this doesn't explain where 3.3V comes from, specifically. I think this rather has to do with CPU core voltages and CPU manufacturers who wished to reduce power/heat, and maybe also clock noise. Lundin‭ about 1 month ago

Yeah for sure in LiPos. If we a had semiconductor designer guy here, I'm sure there's some insight at that level as well. OP got dismissed as this being a stupid question and in reality it's really probably preety deep. It just happens to not be a solve an analog circuit type inquiry. leroy105‭ about 1 month ago

Nobody dismissed anyone...? The question sits at 5 up-votes, 0 down-votes. I'd be curious to learn the reason too but nobody seems to know. Lundin‭ about 1 month ago

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