Can cmos or ttl source more current?

[seyyah]

I have two same purpose logic IC one is 74HCXX series and the other is HCF40XX series. As long as i know ttl ones can source more current than cmos. But in my experiment 74HCXX was not able to drive the load (a resistor capacitor network) where HCF40XX can. Is this result conforms with the families of IC's. Or may there be another problem?

 

[FvM]

Yes, 74HCseries has much higher output current than CD4000 series. This is by design.

 

[Old Nick]

I compiled some design guidelines on logic types from the interfacebus website a while ago. I still find it quite useful when I'm knocking out designs.

 

[Audioguru]

74HCXX is not TTL. It is high-speed Cmos and its output goes to 0V and up to the supply voltage when it has a low load current. Its output high and output low currents are much higher than its 25mA max allowed output current. Its supply is from 2V to 6V.

CD4XXX is ordinary Cmos. With a 5V supply ist max output current high or low is about 5mA. Its supply is from 3V to 18V.

74XX is ordinary TTL. its supply is only 5.0V. Its output goes from 0.1V to about 3.5V when it has a low load current. Its max output low current is 16mA and its max output high current is next to nothing.

74LSXX is low power fast TTL. Its output voltages are about the same as ordinary TTL but its max output currents are lower.

 

[Borber]

Logic chips can be paralleled to increase fanout. Just piggy-back them and solder.

 

[seyyah]

Hmm according to the datasheet 74hcxx can drive a 25mA load where hcf40xx can 10mA. So the problem does not seem to be the drive capability. I simulated the circuit and i faced the same problem. I then realized that the sytem can't start up properly. The chip that i use is a latch(74hc279). Inputs of the latch changes before the output can at the start then it becames locked. If i add a small delay to one of the inputs then it starts up and runs properly in the simulation but i didn't tried on real yet. Although this seems in simulation, the situation is interesting. Because the inputs are also depended with the output. I didn't understand how input could change without the output. Anyway i'll test the real circuit and i hope it responds well.