HC vs LS integrated circuits

[patroclus]

I've seen many schematics were the author says that you must use 74HCxxx and not 74LSxxx. Why is this? I mean, if you design a circuit with only TTL devices, cant you just use all LS?? What are the practical diferences?

(I want to use them for parallel port circuits)
Thanks

 

[Russlk]

74LSxx is made with bipolar transistors and there is a pullup resistor at the input. 74HCxx is made with CMOS transistos and there is no resistance at the input.

 

[patroclus]

I know, and also, high and low voltage input level differ (LS and HCT are TTL, and HC CMOS)... but, how does it affect that in practical circuits?? I mean, why many people recomend HC for some circuits?..

do you think it is better LS for parallel control projects? I thinks so, but I've seen many projects that use HC (or 40 series. Are 40 series the same as HC?)

Thanks

 

[samcheetah]

many people recommend using CMOS ICs because they dissipate less power. CMOS devices have lower switching time so the amount of time the device stays in the active region is less which reduces the amount of power dissipated. if u look at the CMOS inverter and compare it with the bipolar inverter u will see that when the inverter isnt doing anything it is dissipating a little bit of power because a little bit of base current flows. due to that little base current there is a corresponding collector current and when there is collector current there is power being dissipated. in the CMOS inverter the Gate is insulated so there is no flow of charge i.e. no current. and hence no power being dissipated. thats why CMOS devices are preffered where power dissipation should be kept low. and remember that CMOS devices are very sensitive to ESD. this is the simple trade-off principle.

one thing more, in TTL ICs there is a current draw at Low logic level. so if u have connected a TTL input to a low logic level it will be continuously drawing current. this current is about 0.25mA for the LS series. whereas in CMOS devices this is not the case.

i hope that helps!!

 

[Juglenaut]

CMOS's can also drive more gates, etc than LS's..

Fan-out..

Some CMOS's are actually not compatible with LS's unless interfaced or vice versa.

"collector"??? in a IC gates, pardon if I am wrong, but I though that transistor diode family had collectors.

 

[samcheetah]

"collector"??? in a IC gates, pardon if I am wrong, but I though that transistor diode family had collectors

do u mean to say that transistors in the TTL dont have collectors????

 

[spuffock]

You would know the difference if you had to buy the batteries :!:
A project with a 74ls chip might make a battery last, say, a week. The same thing with 74hc might run for 5 years :shock: