The specs (for TI chips) say anywhere from 3 - 18V, and I've measured two alkaline AA batteries at just over 3 volts. I'm wondering if that dips below 3V when the batteries get old, whether or not that would cause problems. Or, if there are other considerations I'm not aware of.
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3\$\begingroup\$ The CD4000 series functions from at least 2.5V to roughly 18V in reality. 3V-18V is the range in which the datasheet's parameters are tested. So you can go lower than 3V but please don't sell this product to anyone. \$\endgroup\$– JankaCommented Oct 13, 2017 at 4:36
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\$\begingroup\$ Alkaline batteries have a very sloped discharge curve. They drop below 1.5V very early in the discharge curve. If you are just goofing around, or designing something with no clocks (combinatorial logic only) it will probably work. But I agree with Janka. \$\endgroup\$– user57037Commented Oct 13, 2017 at 4:41
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\$\begingroup\$ You could use your batteries with a boost converter to get higher voltage, and more useful life from the batteries. \$\endgroup\$– ColinCommented Oct 13, 2017 at 9:15
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3 Answers
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Sure you can run it off 3V, but not nearly for the life of the battery. Ciss and RdsOn both rise quadratically below 3V until they can no longer even drive an input gate. It is silly to choose CD4000 series unless you have no other choice.
You must understand that operating at Vgs(th) means the threshold of conduction and not at a low RdsOn at 3 times this value. Operating below 3V is not only not gauranteed to work with batch tolerances but is rapidly approaching the Vgs threshold or saturation region.
When learning how to be like an Engineer, it means to follow specs and allow a significant marging to avoid failure. There are many other families that work down to 2V
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\$\begingroup\$ I selected this as the answer, because my design constraint is two batteries. I'd use 3 batteries if I could, but that's not an option. So, can you give an example of another family that works below 3V? \$\endgroup\$– KlorthoCommented Oct 14, 2017 at 2:12
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New AA batteries are 1.5V ish, but thet goes down to around 0.9V before they're really empty. I'd use 3 AA batteries if I wanted to get reasonable life out of the batteries.
this also allows operation off NiMh cells.
answered Oct 13, 2017 at 4:52
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Devices typically have two sets of basic electrical parameters
Absolute Maximum Ratings, which represent the values, exceeding which will most likely permanently damage the chip.
Recommended Operating Characteristics, At these values the chip is guaranteed to work at its specified performance values. Of note if these values are exceeded anything could happen, the chip may or may not work, and may or may not suffer a reduction in performance. Quite often these recommended operating conditions are specified to allow full performance over the entire temperature range, and may work ok at slightly beyond the limits at room temperature. This should not be relied upon if designing a product.
