Technical Summary: TTL (Transistor-Transistor Logic)

Overview

Transistor-Transistor Logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJTs) and resistors. It is one of the most widely used logic families in digital electronics, known for its robustness, speed, and compatibility with a variety of applications. TTL circuits are characterized by their use of multi-emitter transistors in earlier designs, though modern variants like LS (Low-Power Schottky) and FAST (Fairchild Advanced Schottky TTL) have evolved to use improved input structures such as Schottky diodes for enhanced performance.

The TTL family includes several sub-families, each optimized for specific performance metrics such as power consumption, speed, or noise immunity. Among these, LS and FAST TTL are prominent due to their balanced trade-offs between speed and power efficiency. The internal structure of TTL gates, particularly the 2-input NAND gate, serves as a foundational example to understand the operational principles of the entire family.

Features

The key features of TTL logic, particularly the LS and FAST families, are highlighted by their circuit design and performance characteristics:

• Improved Input Configuration: Unlike traditional TTL circuits that use multi-emitter input structures, Motorola LS TTL circuits employ a DTL (Diode-Transistor Logic) type input with Schottky diodes (e.g., D3 and D4 in Figure 2-1a). This design enhances speed and increases input breakdown voltage.
• High Input Breakdown Voltage: The input breakdown voltage for LS TTL circuits is typically 15V, with leakage tests conducted at 7.0V applied input voltage.
• Schottky Clamping: FAST TTL utilizes Schottky diodes (e.g., D5-D10 in Figure 2-1b) to prevent transistor saturation, reducing propagation delay and improving switching speed.
• Output Stage Design: The output stages of LS and FAST TTL gates (e.g., Q4 and Q3 in LS00, Q4-Q6 and Q9 in F00) are optimized for drive capability and noise immunity.
• Power Efficiency: LS TTL offers lower power consumption compared to standard TTL, while FAST TTL provides higher speed without excessive power dissipation.

Circuit Details

The internal schematics of a 2-input NAND gate in LS and FAST TTL (Figures 2-1a and 2-1b) reveal the following design elements:

• LS00 (LS TTL 2-Input NAND Gate): Features a DTL-style input with Schottky diodes (D3, D4), a phase-splitter transistor (Q2), and a totem-pole output stage (Q3, Q4). Resistor values (e.g., 3.5K, 15K) are carefully selected to balance speed and power.
• F00 (FAST TTL 2-Input NAND Gate): Incorporates additional Schottky diodes (D5-D10) and a more complex output stage (Q5, Q6, Q9) to achieve faster switching. The use of lower resistor values (e.g., 2K, 5K) further enhances speed.

Applications

TTL logic is widely used in both legacy and modern digital systems due to its reliability and ease of integration. Key applications include:

• Digital Computing: TTL circuits form the backbone of many early microprocessors, ALUs, and memory systems.
• Industrial Control Systems: Used in PLCs (Programmable Logic Controllers) and automation equipment due to their noise immunity.
• Communication Devices: TTL interfaces are common in serial communication protocols like RS-232 and UART.
• Test and Measurement Equipment: Logic analyzers, pulse generators, and oscilloscopes often utilize TTL-compatible signals.
• Consumer Electronics: Found in legacy gaming consoles, calculators, and embedded systems.

Electrical Characteristics

The electrical performance of LS and FAST TTL is summarized below:

LS TTL (e.g., LS00)

• Supply Voltage (V_CC): 4.75V to 5.25V
• Propagation Delay: 9-15 ns
• Power Dissipation per Gate: 2 mW (typical)
• Input Leakage Current: < 0.1 µA at 7.0V input
• Output Drive Capability: 8 mA (sink), 0.4 mA (source)

FAST TTL (e.g., F00)

• Supply Voltage (V_CC): 4.75V to 5.25V
• Propagation Delay: 3-6 ns
• Power Dissipation per Gate: 4 mW (typical)
• Input Leakage Current: < 0.1 µA at 7.0V input
• Output Drive Capability: 20 mA (sink), 1 mA (source)

Package Information

TTL ICs are available in multiple package types, catering to different assembly and thermal requirements:

• DIP (Dual In-line Package): 14-pin or 16-pin configurations for through-hole mounting (e.g., LS00 in PDIP-14).
• SOIC (Small Outline IC): Surface-mount variant for compact PCB designs (e.g., F00 in SOIC-14).
• TSSOP (Thin Shrink Small Outline Package): Ultra-thin profile for high-density applications.
• Ceramic Packages: Used in military or high-reliability systems for superior thermal performance.

Pin Configuration (LS00 Example)

• Pin 1: Input A (Gate 1)
• Pin 2: Input B (Gate 1)
• Pin 3: Output Y (Gate 1)
• Pin 4: Input A (Gate 2)
• Pin 5: Input B (Gate 2)
• Pin 6: Output Y (Gate 2)
• Pin 7: Ground (GND)
• Pin 8: Output Y (Gate 3)
• Pin 9: Input B (Gate 3)
• Pin 10: Input A (Gate 3)
• Pin 11: Output Y (Gate 4)
• Pin 12: Input B (Gate 4)
• Pin 13: Input A (Gate 4)
• Pin 14: Supply Voltage (V_CC)