Texas Instruments SN74HC138DT

Overview

SN74HC138DT from Texas Instruments is a 3-line to 8-line decoder/demultiplexer with active-low outputs, designed for high-speed memory decoding and data-routing applications. It operates across 2 V to 6 V, delivers 15 ns typical propagation delay at 5 V, supports ±4-mA output drive, and draws ≤80 µA ICC - enabling low-power, high-density address decoding in industrial control and embedded systems.

For engineers reviewing the SN74HC138DT datasheet, SN74HC138DT pinout, SN74HC138DT application, or SN74HC138DT equivalent, this device serves as a core logic element in LED matrix column selection, memory enable gating, and I/O expansion where precise 3-bit binary-to-1-of-8 decoding with three enable inputs is required.

Technical Context

The SN74HC138DT implements CMOS-based 3-to-8 decoding with three binary select inputs (A, B, C) and three enable terminals: one active-high (G1) and two active-low (G2A, G2B). All outputs are active-low and mutually exclusive - only one Y0–Y7 is asserted low per valid input combination when all enables are satisfied.

Its functional mode relies on strict enable hierarchy: outputs remain high unless G1 = HIGH, G2A = LOW, and G2B = LOW simultaneously. This architecture simplifies cascading into larger decoders (e.g., 24-line using no external inverters) and supports demultiplexing by repurposing an enable pin as data input.

Key Specifications

Pinout & Package

SN74HC138DT uses a 16-pin SOIC (D) package with nominal body size 9.90 mm × 3.90 mm and standard 1.27-mm lead pitch. Pin numbering follows JEDEC MS-012, with pins 1–16 arranged in dual in-line configuration.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138DT, SN74HCT138DR offers guaranteed TTL input compatibility at the cost of higher quiescent current, while 74AC138SCX delivers superior speed and drive strength but demands stricter signal integrity controls - making SN74HC138DT the optimal balance of industrial temperature range, low power, and broad voltage flexibility.

Availability

SN74HC138DT is available at Aetrix Electronics and suitable for industrial control panels, LED display subsystems, and factory automation hardware requiring stable component supply across extended product lifecycles.

Supply support for SN74HC138DT 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 solutions with over 50 years of innovation in industrial and automotive electronics.

The SN74HC138DT belongs to TI's HC (High-Speed CMOS) logic family, engineered for low-power, wide-voltage-operation digital interfacing in noise-resilient industrial environments.

FAQ

What is the operating voltage range for SN74HC138DT?

The SN74HC138DT operates from 2 V to 6 V DC, supporting both 3.3-V and 5-V logic systems without external level translation. Its input thresholds scale with VCC - VIH minimum is 1.5 V at 2 V supply and 4.2 V at 6 V - ensuring reliable switching across the full range. This makes SN74HC138DT suitable for mixed-voltage designs where legacy and modern peripherals coexist on the same board.

Does SN74HC138DT support cascading for larger decoder configurations?

Yes, SN74HC138DT supports seamless cascading using its three enable inputs: G1 (active-high), G2A, and G2B (both active-low). By connecting outputs of one SN74HC138DT to enables of others, designers can build 24-line decoders without external inverters. For example, three SN74HC138DTs can decode 5 address bits (A0–A4) into 24 unique outputs - a capability explicitly documented in TI's SCLS107G datasheet.

What is the maximum output sink current for SN74HC138DT?

SN74HC138DT guarantees ±4 mA output drive at VCC = 4.5 V, with VOL ≤ 0.26 V under that load. At 6 V, it sustains ±5.2 mA with VOL ≤ 0.26 V. These values allow direct driving of up to 10 LSTTL loads per output. Exceeding these limits risks violating VOL specs or exceeding absolute maximum ratings - so external buffers are recommended for >10-LSTTL fan-out or higher-current loads like LEDs without series resistors.

Is SN74HC138DT pin-compatible with other 74HC138 variants?

Yes, SN74HC138DT shares identical pinout and function with all SOIC-packaged 74HC138 variants (e.g., SN74HC138DR, SN74HC138DRE4) and matches the D-package footprint defined in JEDEC MS-012. However, it is not pin-compatible with SSOP (DB), TSSOP (PW), or PDIP (N) versions due to differing package dimensions and lead spacing - though logic behavior and terminal functions remain consistent across all packages.

How does the enable logic work on SN74HC138DT?

SN74HC138DT requires all three enables to be simultaneously active: G1 must be HIGH, and both G2A and G2B must be LOW. If any enable is inactive, all eight outputs (Y0–Y7) go HIGH - regardless of A/B/C state. This triple-enable structure allows flexible system-level control: G1 may connect to a processor's chip-select, while G2A/G2B tie to board-level address or mode signals, enabling selective activation of decoder banks.