74138 - 3-to-8 Line Decoder/Demultiplexer

Overview

The 74138 is a high-speed, 3-to-8 line decoder/demultiplexer integrated circuit designed for digital logic applications. This device accepts a 3-bit binary input and decodes it into one of eight mutually exclusive outputs, which are active LOW. The 74138 features three enable inputs (two active LOW and one active HIGH) that allow for easy expansion to larger decoding configurations, such as 4-to-16 or 5-to-32 line decoders using multiple devices.

This component is widely used in memory address decoding, data routing, and other applications where binary information needs to be converted into individual control signals. The 74138 is part of the 7400 series of TTL logic devices and is known for its reliability and performance in digital systems.

Features

• High-Speed Operation: Typical propagation delay of 21ns at 5V.
• Wide Operating Voltage Range: 4.75V to 5.25V.
• Low Power Consumption: Typically 32mW power dissipation.
• 3-to-8 Decoding: Converts 3 binary inputs into 8 mutually exclusive outputs.
• Multiple Enable Inputs: Two active LOW (G2A, G2B) and one active HIGH (G1) for flexible control.
• Active LOW Outputs: All eight outputs are active LOW for easy interfacing with other logic.
• TTL Compatible: Fully compatible with all TTL families.
• Demultiplexing Capability: Can function as a 1-to-8 demultiplexer when used with enable inputs.

Applications

The 74138 finds use in a wide variety of digital systems and applications, including:

• Memory Address Decoding: Used in microprocessor systems to decode memory addresses for RAM, ROM, and I/O devices.
• Data Routing: Functions as a data distributor in digital systems.
• Function Generation: Can be used to generate any 3-variable logic function in combination with other gates.
• System Expansion: Multiple 74138s can be cascaded to create larger decoders (4-to-16, 5-to-32, etc.).
• Seven-Segment Display Driving: Part of circuits that drive numeric displays.
• Control Logic: Used in various control applications where specific outputs need to be activated based on input combinations.

Electrical Characteristics

Absolute Maximum Ratings

• Supply Voltage (Vcc): 7V
• Input Voltage: 5.5V
• Operating Temperature Range: 0°C to +70°C (commercial grade)
• Storage Temperature Range: -65°C to +150°C

Recommended Operating Conditions

• Supply Voltage (Vcc): 4.75V to 5.25V
• High-Level Input Voltage (VIH): 2V min
• Low-Level Input Voltage (VIL): 0.8V max
• Output Current: 8mA (HIGH), 16mA (LOW)

DC Characteristics (Vcc = 5V, TA = 25°C)

• Input Clamp Voltage (VIK): -1.5V max at IIK = -18mA
• High-Level Output Voltage (VOH): 2.4V min at IOH = -400µA
• Low-Level Output Voltage (VOL): 0.4V max at IOL = 16mA
• Input Current (II): 1.6mA max at Vcc = 5.5V
• Supply Current (ICC): 4mA max (all outputs HIGH), 8mA max (all outputs LOW)

AC Characteristics

• Propagation Delay (Input to Output): 21ns typical, 30ns max
• Enable Time (G1 to Output): 23ns typical, 32ns max
• Disable Time (G2 to Output): 21ns typical, 30ns max
• Output Transition Time: 15ns typical, 22ns max

Package Information

The 74138 is available in several package options to suit different application requirements:

• 16-pin DIP (Dual In-line Package): The traditional through-hole package with 0.3" row spacing.
• 16-pin SOIC (Small Outline Integrated Circuit): Surface-mount package with 0.05" pin spacing.
• 16-pin TSSOP (Thin Shrink Small Outline Package): Smaller surface-mount option for space-constrained applications.

Pin Configuration

• Pin 1: G2A (Enable Input, active LOW)
• Pin 2: G2B (Enable Input, active LOW)
• Pin 3: G1 (Enable Input, active HIGH)
• Pin 4: A (Address Input, LSB)
• Pin 5: B (Address Input)
• Pin 6: C (Address Input, MSB)
• Pin 7: Y0 (Output 0, active LOW)
• Pin 8: GND (Ground)
• Pin 9: Y1 (Output 1, active LOW)
• Pin 10: Y2 (Output 2, active LOW)
• Pin 11: Y3 (Output 3, active LOW)
• Pin 12: Y4 (Output 4, active LOW)
• Pin 13: Y5 (Output 5, active LOW)
• Pin 14: Y6 (Output 6, active LOW)
• Pin 15: Y7 (Output 7, active LOW)
• Pin 16: Vcc (Positive Supply, +5V)

Functional Description

The 74138 operates as a 3-to-8 decoder when all enable inputs are properly asserted (G1 HIGH, G2A and G2B LOW). The binary value applied to inputs C (MSB), B, and A (LSB) determines which of the eight outputs (Y0-Y7) will be active (LOW). All other outputs remain HIGH. The truth table below illustrates the complete functionality:

Truth Table

• Enable Inputs: G1=H, G2A=L, G2B=L (