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Registered Member #1157
Joined: Thu Dec 06 2007, 12:11PM
Location: Houston, TX
Posts: 307
Looking for Hex inverters the other day, I came across the 74LS14. This is the predecessor of the CMOS 74HC14, but with tighter voltage requirements. The 74HC14 can be run anywhere from 2.0 to 6 VDC. To me the chips do the same thing, but a question came up that the switching speeds might not be as fast with the TTL LS inverter as with the HC inverter.
Looking it up on Wikipedia, it doesn't mention speed differences between the LS and HC families, only that the voltage requirements were looser with the HC.
My question is, other than the tight voltage requirement of the LS chip (4.75 to 5.25 Vdc), is there much difference in the actual function/speed of the two logic families?
Reading the data sheets, I don't really see a difference in the speeds, but the LS doesn't really list them.
I'm primarily using the chip with the antenna on my version of Steve's Mini, but I was also going to invert the input of one of my gate driver chips since I found a military spec chip for cheap and they didn't have the inverting version.
Banned on 3/17/2009. Registered Member #487
Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617
The HC parts are cmos and work quite different. The HC versions can run to 50MHz. Not sure about the LS version. The LS version from what I remember were basically like the TTL but didn't suck as much power. LS stands for low power shottky HC is high speed CMOS.
I haven't tried an LS version on a coil before but im sure it would work. I have tried 74AC14 though and it works too. Any inverter or buffer schmitt or not will work for that. I am pretty sure the LS series wont work for things like oscillators but I could be wrong. If you're using it for an SSTC then I would go for the newer 74HC14 even though all of them will work for that but if you need it for something else then you're probably better off with the HC. Or better yet I like using the CD40106 because it can run up to 18 volts which means your driver chips and your logic can all run off the same regulator.
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
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Posts: 253
Output swing on LS will be lower - they don't go up as high. HC is better than LS in every important respect, which is why LS has been obsolete for many years. LS became very popular in the 80'sas it brought TTL power consumption down to a manageable level for use on dense boards, and was fast enough to keep up with the 8-bit processors of the day, but once HC became widely available, it replaced LS pretty quickly.
Since 74LS14 is TTL compatible the replacement CMOS part would be 74HCT14. It has the same voltage requirements as 74LS14. They have those requirements, not for function, but for correct TTL input/output levels. The 74HC chip will require a higher input voltage than TTL compatible chips. So a HC chip can drive a LS chip, but not the other way round unless the HC chip is run on a lower voltage than 5V.
The 74HC ones tend to be faster, but it depends on the voltage. The main difference is that the 74LS needs an input current and the 74HC(T) does not need a current except what is needed to overcome parasitic capacitance/resistance.
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253
Bjørn wrote ...
74HCT14 = High-speed CMOS with TTL compatible output levels
HCT has TTL-level input levels - about 0.8v vs. 1/3 and 2/3VCC, and are only specced for 5V supplies. They are useful for converting 3/3.3v levels to 5V, as 3.3v levels are marginal for HC. Minor caveat is that the power draw is higher when not driving the inputs to the rails - only an issue when you care about microamps though.
Registered Member #1157
Joined: Thu Dec 06 2007, 12:11PM
Location: Houston, TX
Posts: 307
You guys Rock! These were the answers I was looking for, and I now feel I have a better understanding of what the chips will do in my SSTC.
Tom, good call on that CD40106.
On the next board I prototype, I'll retool the board to have only one regulator, saving space, money and time. BTW, How is work going on those boards you were designing to fit in the ATX power supply case?
The SSTC I'm prototyping now fits into an ATX case and even uses the same fan for cooling.
Banned on 3/17/2009. Registered Member #487
Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617
I haven't worked on them since but I will begin messing with them here shortly. I have a repaired board ready to go. Problem was I was pushing that thing too hard. his time I'm using gate drive Ic's.
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Difference in Logic Families
