5V->3V IC selection question

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thomedwards
 
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5V->3V IC selection question

Post by thomedwards »

I'm curious as to why one would choose the 74AHC125N over the CD4050 (or vice versa) for 5V -> 3V3 conversion. Is it project specific, or are they generally interchangeable? I see that both chips are used in different adafruit projects, and wondered why. They are listed in the parfinder wiki as being used for the same purpose, but there must be a reason for one over the other.

Also, if someone was looking for similar chips, what are the key words that would be used for searching? buffer, non-inverting?

Also, why is 74AHC125N considered tri-state? What characteristics make it called that? Likewise, why is the CD4050 6 channel? (I assume because it can handle six I/O, but I want to make sure.)

I know this might include quite a bit of ground and is probably a very novice question, but I have found myself very interested in the component selection and circuit design of many projects here, and less on just soldering them up for Arduino use. Even just some links to the answers if you don't have the energy/motivation to try to answer here would be great. I've searched around some, but I feel like I don't know enough to create a search that gives me the answers I'm looking for.

Thanks for any and all responses,
Thom

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zener
 
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Re: 5V->3V IC selection question

Post by zener »

The 4050 has 6 seperate buffers so it is called "hex" or "6 channel". Its outputs can be high or low. The outputs of the HC/AHC125 can have a third state which is "disconnected". This is also known as "high impedance". In this third state they let go of the line so that some other device can control it. It is activated by the extra line going to the side of the gate in the schematic. Technically "Tri-State" is a registered trademark so not all mfg call it that. There are many parts that do the same thing. That is why you see different parts being used. Sometimes the designer is looking at some particular spec that makesd them lean one way or the other, or they just use what they used in the past.

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ImaginaryAxis
 
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Re: 5V->3V IC selection question

Post by ImaginaryAxis »

When a device is tri-stated the output transistors are turn off. In CMOS devices, the gate of the output pMOS is taken to VCC and the gate of the output nMOS is taken to GND. This places the transistors in cutoff and they are, quite literally, high impedance through the channel. Since the outputs are in high impedance they draw no current and will allow a voltage signal to be driven at their outputs with no ill effects. They are disconnected from the line or bus like Zener said.

Tri-state
Hi-Z
Floating node

Can be interchangeable.

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