Texas Instruments SN74HCT138DRE4

Overview

SN74HCT138DRE4 from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in SOIC-16 package, operating at 4.5–5.5 V with 17 ns typical propagation delay, ±4-mA output drive, and TTL-compatible inputs. It serves as a high-speed memory address decoder or data routing switch in embedded control and digital logic systems.

For engineers reviewing the SN74HCT138DRE4 datasheet, SN74HCT138DRE4 pinout, SN74HCT138DRE4 application, or SN74HCT138DRE4 equivalent, key selection criteria include enable-input logic configuration (two active-low, one active-high), output drive capability under LSTTL load conditions, propagation delay vs. memory access timing, and SOIC-16 thermal performance (R_θJA = 73°C/W).

Technical Context

The SN74HCT138DRE4 implements combinational logic decoding using three binary select inputs (A, B, C) and three enable terminals (G1, G2A, G2B) to activate exactly one of eight active-low outputs (Y0–Y7). Its HCT logic family ensures TTL-voltage compatibility while maintaining CMOS power efficiency.

Propagation delay is specified at 17 ns (typical) under 5.5 V and 25°C with 50-pF load, and switching behavior is validated across temperature (–40°C to +85°C) and supply voltage (4.5–5.5 V). The device supports cascading via enable inputs without external inverters-e.g., a 24-line decoder requires no additional gates.

Key Specifications

Pinout & Package

SN74HCT138DRE4 uses a 16-pin SOIC (D) package with 1.27-mm pitch, 9.90 mm × 3.90 mm body size, and 73°C/W junction-to-ambient thermal resistance. Pin 1 is located at the top-left corner with notch or dot marking.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 3-to-8 decoder applications.

Compared with SN74HC138DR and 74LV138PW, SN74HCT138DRE4 uniquely delivers TTL-compatible input thresholds at 5 V while maintaining low ICC and predictable 17-ns timing-making it the optimal choice for retrofitting or maintaining legacy 5-V digital systems without signal integrity compromises.

Availability

SN74HCT138DRE4 is available at Aetrix Electronics and suitable for industrial control panels, memory subsystems, and programmable logic interfaces requiring stable component supply and long-term manufacturability.

Supply support for SN74HCT138DRE4 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a global semiconductor leader specializing in analog, embedded processing, and logic ICs, with over 90 years of innovation in industrial, automotive, and communications markets.

The SN74HCT138DRE4 belongs to TI's HCT logic family-designed specifically for high-speed memory decoding and data-routing applications where TTL compatibility, low power, and precise propagation timing are critical.

FAQ

What is the maximum operating temperature range for SN74HCT138DRE4?

The SN74HCT138DRE4 is rated for operation from –40°C to +85°C, meeting industrial temperature requirements. This range is validated across all electrical specifications including propagation delay, output drive, and input thresholds, ensuring reliable performance in embedded controllers, motor drives, and factory automation equipment where ambient temperatures fluctuate significantly. The SOIC-16 package's R_θJA of 73°C/W supports thermal management without forced cooling in most applications.

Does SN74HCT138DRE4 require external pull-up resistors on its outputs?

No, SN74HCT138DRE4 does not require external pull-up resistors because its outputs are active-low push-pull drivers capable of sinking 4 mA and sourcing 4 mA. Each Y0–Y7 output can directly interface with TTL or CMOS inputs without passive components. Pull-ups would interfere with the intended active-low assertion logic and increase power consumption unnecessarily-TI's recommended application circuits confirm direct connection to chip-select or enable inputs.

Can SN74HCT138DRE4 be used as a demultiplexer?

Yes, SN74HCT138DRE4 functions as a 1-to-8 demultiplexer when one enable input (e.g., G1) is used as the data input while A, B, C serve as address selectors. With G2A and G2B held LOW and G1 driven by the incoming signal, the selected Yx output mirrors the G1 state (inverted due to active-low output). This configuration is explicitly supported in TI's functional description and enables simple serial-to-parallel data distribution in communication interfaces or test equipment.

What is the recommended bypass capacitor for SN74HCT138DRE4?

Texas Instruments recommends a 0.1-µF ceramic bypass capacitor placed as close as possible to the V_CC and GND pins of SN74HCT138DRE4. This value is specified in Section 8 of the datasheet to suppress high-frequency noise and stabilize the supply during output switching transients. For systems with broad-spectrum noise, paralleling a 1-µF capacitor is acceptable-but the 0.1-µF unit must remain adjacent to the IC to ensure effective decoupling at the SOIC-16 lead inductance.

How many SN74HCT138DRE4 devices are needed to decode a 5-bit address into 32 outputs?

Two SN74HCT138DRE4 devices are required to decode a 5-bit address into 32 outputs: the upper two bits (A4, A3) control the enable inputs of two SN74HCT138DRE4 units, while the lower three bits (A2, A1, A0) feed the select inputs of both. This configuration-documented in TI's application notes-uses G1, G2A, and G2B to partition the 32-line space without external inverters, leveraging the device's native enable architecture for efficient hierarchical decoding.