74LS595 verses 74HC595

Eric Adams

I have been using 74LS595 as LED Display drivers at 15ma per 595 output.· These 595's work with the SX28.· I previously noticed in the Jameco Catalog a vast price difference in the HC595 $.62 and the LS595 $4.55 .· The pinouts are the same so I ordered a few to see if they work, and they did.· However I lack the expertise to determine from the data sheets what the difference is and if there are any drawbacks to using the HC verses the LS for projects.

Any help appreciated.

Thanks

CPUMAN

LS·is TTL based, fast, uses more power and·is older.
HC is CMOS based, fast, uses less power and is the most up-to-date version.

So my guess is the TTL design is more expensive to build.· Maybe someone knows more about the costs of building a TTL chip vs a CMOS chip.

Here's a site I dug up off the net that seems to have fairly good explinations.

http://www.kpsec.freeuk.com/components/74series.htm

Chris

Paul Baker

The reason LS series parts is getting so expensive is that they are generally considered obsolete, so new designs aren't using them and the supply-demand curve pushes up the price.

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Nate

Hi Eric,

Short answer to your question; it is fine to use the HC (high Speed CMOS) series logic with the SX.· This is one of the few times in life you actually do get more for less.· A quick perusal of the data sheet shows that the devices are roughly equivilent, but the HC version has a much higher allowable clock frequency (100MHz vs 20MHz), this is good as far as the super fast SX is concerned.

As others have stated, the LS series is based on TTL (total transistor logic) which means that·thay are made with·bipolar transisitors (BJT).· BJT's were·one of the first transistors used in ICs.

The HC series use CMOS (complementary metal oxide semiconductor) devices·that are a later generation transistor that use, as the name denotes, metal over an oxide as the gate in the transistor (nowdays it's polysilicon over an oxide).· The advantage to these devices·is that the input impedance is much higher than the BJT.· This translates into much lower current and power usage.

As a simple rule (which has exceptions), CMOS is used in logic devices where the low power·needs are good, and BJTs are used in higher power devices (switching, power management apps., etc).· There are also BiCMOS devices that put both CMOS and BJTs on the same die and use the best atributes·of·each device.

The major reason for the price difference is as Paul said, supply and demand.· Well actually, supply.· The HC series has about 8 bazillion made per year, while there are only a piddling of the LS series produced per year.· So use the HC series and laugh at the fact that you are getting more for less.

Nate

Eric Adams

Thanks to all of you and especially Nate for his detailed reply, which I understood. Seems like a win/win situation on these HC 595's.

Again Thanks