Monday, 7 July 2014

DIFFERENCES IN CMOS 4000 SERIES, 74LS, 74HC, 74HCT SERIES IC

DIFFERENCES OF CMOS 4000 SERIES, 74LS, 74HC, 74HCT SERIES IC

Integrated Circuit (IC) is fastly growing and different technologies have been developed and being developed. Most commonly available IC families are CMOS 4000, 74LS, 74HC, 74HCT. Now let us see the characteristics of those families and their differences.



4000 Series CMOS
        This family of logic ICs is numbered from 4000 onwards, and from 4500 onwards. They have a B at the end of the number (e.g. 4001B) which refers to an improved design introduced some years ago. Most of them are in 14-pin or 16-pin packages. They use CMOS circuitry which means they use very little power and can tolerate a wide range of power supply voltages (3 to 15V) making them ideal for battery powered projects. CMOS is pronounced 'see-moss' and stands for Complementary Metal Oxide Semiconductor. 

       However the CMOS circuitry also means that they are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them. For the more sensitive (and expensive!) ICs special equipment is available, including earthed wrist straps and earthed work surfaces.

74LS, 74HC, 74HCT
There are several families of logic ICs numbered from 74xx00 onwards with letters (xx) in the middle of the number to indicate the type of circuitry, eg 74LS00 and 74HC00. The original family (now obsolete) had no letters, eg 7400. 

The 74LS (Low-power Schottky) family (like the original) uses TTL(Transistor-Transistor Logic) circuitry which is fast but requires more power than later families. The 74 series is often still called the 'TTL series' even though the latest ICs do not use TTL! 

The 74HC family has High-speed CMOS circuitry, combining the speed of  TTL with the very low power consumption of the 4000 series. They are CMOS ICs with the same pin arrangements as the older 74LS family. Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible, use 74HCT instead.

The 74HCT family is a special version of 74HC with 74LS TTL-compatible inputs so 74HCT can be safely mixed with 74LS in the same system. In fact 74HCT can be used as low-power direct replacements for the older 74LS ICs in most circuits. The minor disadvantage of 74HCT is a lower immunity to noise, but this is unlikely to be a problem in most situations. 

The CMOS circuitry used in the 74HC and 74HCT series ICs means that they are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them. 

CMOS 4000 series family characteristics:
  • Supply: 3 to 15V, small fluctuations are tolerated.
  • Inputs have very high impedance (resistance), this is good because it means they will not affect the part of the circuit where they are connected. However, it also means that unconnected inputs can easily pick up electrical noise and rapidly change between high and low states in an unpredictable way. This is likely to make the IC behave erratically and it will significantly increase the supply current. To prevent problems all unused inputs MUST be connected to the supply (either +Vs or 0V), this applies even if that part of the IC is not being used in the circuit!
  • Outputs can sink and source only about 1mA if you wish to maintainthe correct output voltage to drive CMOS inputs. If there is no need to drive any inputs the maximum current is about 5mA with a 6V supply, or 10mA with a 9V supply (just enough to light an LED). To switch larger currents you can connect a transistor.
  • Fan-out: one output can drive up to 50 inputs.
  • Gate propagation time: typically 30ns for a signal to travel through a gate with a 9V supply, it takes a longer time at lower supply voltages.
  • Frequency: up to 1MHz, above that the 74 series is a better choice.
  • Power consumption (of the IC itself) is very low, a few µW. It is much greater at high frequencies, a few mW at 1MHz for example.

74HC and 74HCT family characteristics:
  • 74HC Supply: 2 to 6V, small fluctuations are tolerated.
  • 74HCT Supply: 5V ±0.5V, a regulated supply is best.
  • Inputs have very high impedance (resistance), this is good because it means they will not affect the part of the circuit where they are connected. However, it also means that unconnected inputs can easily pick up electrical noise and rapidly change between high and low states in an unpredictable way. This is likely to make the IC behave erratically and it will significantly increase the supply current. To prevent problems all unused inputs MUST be connected to the supply (either +Vs or 0V), this applies even if that part of the IC is not being used in the circuit! Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible. For reliability use 74HCT if the system includes some 74LS ICs.
  • Outputs can sink and source about 4mA if you wish to maintain the correct output voltage to drive logic inputs, but if there is no need to drive any inputs the maximum current is about 20mA. To switch larger currents you can connect a transistor.
  • Fan-out: one output can drive many inputs (50+), except 74LS inputs because these require a higher current and only 10 can be driven.
  • Gate propagation time: about 10ns for a signal to travel through a gate.
  • Frequency: up to 25MHz.
  • Power consumption (of the IC itself) is very low, a few µW. It is much greater at high frequencies, a few mW at 1MHz for example. 

74LS family TTL characteristics:
  • Supply: 5V ±0.25V, it must be very smooth, a regulated supply is best. In addition to the normal supply smoothing, a 0.1µF capacitor should be connected across the supply near the IC to remove the 'spikes' generated as it switches state, one capacitor is needed for every 4 ICs.
  • Inputs 'float' high to logic 1 if unconnected, but do not rely on this in a permanent (soldered) circuit because the inputs may pick up electrical noise. 1mA must be drawn out to hold inputs at logic 0. In a permanent circuit it is wise to connect any unused inputs to +Vs to ensure good immunity to noise.
  • Outputs can sink up to 16mA (enough to light an LED), but they can source only about 2mA. To switch larger currents you can connect a transistor.
  • Fan-out: one output can drive up to 10 74LS inputs, but many more 74HCT inputs.
  • Gate propagation time: about 10ns for a signal to travel through a gate.
  • Frequency: up to about 35MHz (under the right conditions).
  • Power consumption (of the IC itself) is a few mW.

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