TTL Families

TTL Families

TTL families have evolved over the years in response to the demands of digital designers for better performance. All of the TTL families are compatible in that they use the same power supply voltage and logic levels, but each family has its own advantages in terms of speed, power consumption, and cost.

Early TTL Families

The original TTL family of logic gates was introduced by Sylvania in 1963, popularized by Texas Instruments, whose “7400-series” part numbers for gates and other TTL components quickly became an industry standard. As in 7400-series CMOS, devices in a given TTL family have part numbers of the form 74FAMnn, where “FAM” is an alphabetic family mnemonic and nn is a numeric function designator. Devices in different families with the same value of nn perform the same function. In the original TTL family, “FAM” is null and the family is called 74-series TTL.

Resistor values in the original TTL circuit were changed to obtain two more TTL families with different performance characteristics.

The 74H (High speed TTL) family used lower resistor values to reduce propagation delay at the expense of increased power consumption.

The 74L (Low-power TTL) family used higher resistor values to reduce power consumption at the expense of propagation delay.

The availability of three TTL families allowed digital designers in the 1970s to make a choice between high speed and low power consumption for their circuits. The development of Schottky transistors provided an opportunity for combining all three TTL, and made 74, 74H, and 74L TTL obsolete.

Schottky TTL Families

1.   Historically, the first family to make use of Schottky transistors was 74S (Schottky TTL). With Schottky transistors and low resistor values, this family has much higher speed, but higher power consumption, than the original 74-series TTL.

2.   74LS - Perhaps the most widely used and certainly the least expensive TTL family is 74LS (Low-power Schottky TTL), introduced shortly after 74S. By combining Schottky transistors with higher resistor values, 74LS TTL matches the speed of 74-series TTL but has about one-fifth of its power consumption. Thus, 74LS is a preferred logic family for new TTL designs.

3.   Subsequent IC processing and circuit innovations gave rise to two more Schottky logic families. The 74AS (Advanced Schottky TTL) family offers speeds approximately twice as fast as 74S with approximately the same power consumption.

4.   The 74ALS (Advanced Low-power Schottky TTL) family offers both lower power and higher speeds than 74LS, and rivals 74LS in popularity for general-purpose requirements in new TTL designs.

5.   The 74F (Fast TTL) family is positioned between 74AS and 74ALS in the speed/power tradeoff, and is probably the most popular choice for high-speed requirements in new TTL designs.

Characteristics of TTL Families

The important characteristics of contemporary TTL families are shown in Table.1. The first two rows of the table list the propagation delay (in nanoseconds) and the power consumption (in milliwatts) of a typical 2-input NAND gate in each family.

Table.1 Characteristics of Gates in TTL Families

  

The figure of merit of a logic family is its speed-power product is simply the product of the propagation delay and power consumption of a typical gate. The speed-power product measures a sort of efficiency—how much energy a logic gate uses to switch its output.

The remaining rows describe the input and output parameters of typical TTL gates in each of the families. Using this information, we can analyze the external behavior of TTL gates without knowing the details of the internal TTL circuit design.

A TTL Data Sheet

Table.2 shows the part of a typical manufacturer’s data sheet for the 74LS00. The 54LS00 listed in the data sheet is identical to the 74LS00, except that it is specified to operate over the full “military” temperature and voltage range, and it costs more. Most TTL parts have corresponding 54-series (military) versions.

Three sections of the data sheet are shown in the table:

Recommended operating conditions specify power-supply voltage, input voltage ranges, DC output loading, and temperature values under which the device is normally operated.

Electrical characteristics specify additional DC voltages and currents that are observed at the device inputs and output when it is operated under the recommended conditions:

II - Maximum input current for a very high HIGH input voltage.

IOS - Output current with HIGH output shorted to ground.

ICCH - Power-supply current when all outputs (on four NAND gates) are HIGH. (The number given is for the entire package, which contains four NAND gates, so the current per gate is one-fourth of the specified amount.)

ICCL - Power-supply current when all outputs (on four NAND gates) are LOW.

Table.2 Typical Manufacturer’s data sheet for the 74LS00

Switching characteristics give maximum and typical propagation delays under “typical” operating conditions of VCC = 5 V and TA = 25°C. A conservative designer must increase these delays by 5%–10% to account for different power-supply voltages and temperatures, and even more under heavy loading conditions.

Absolute maximum ratings indicate the worst-case conditions for operating or storing the device without damage i.e., included as a fourth section in the manufacturer’s data book:

A complete data book also shows test circuits that are used to measure the parameters when the device is manufactured, and graphs that show how the typical parameters vary with operating conditions such as power-supply voltage (VCC), ambient temperature (TA), and load (RL, CL).