Texas Instruments SN74ALS139NSR

Overview

SN74ALS139NSR from Texas Instruments is a dual 2-line-to-4-line decoder/demultiplexer in SOP-16 (NS) package, designed for high-speed memory decoding and data routing. It features Schottky-clamped TTL logic, 3–14 ns propagation delay (tPLH/tPHL), 8 mA low-level output drive, and operates from 0°C to 70°C with 4.5–5.5 V supply. Used in address decoding for SRAM systems where minimal system delay is critical.

For engineers reviewing the SN74ALS139NSR datasheet, SN74ALS139NSR pinout, SN74ALS139NSR application, or SN74ALS139NSR equivalent, key selection criteria include propagation delay budgeting, enable-controlled demux operation, TTL-compatible input thresholds (VIL = 0.8 V, VIH = 2 V), and SOP-16 thermal and layout compatibility in legacy industrial control backplanes.

Technical Context

The SN74ALS139NSR integrates two independent decoders, each with active-low enable (1G/2G) and dual select inputs (A/B), supporting cascaded decoding via enable chaining. Its fully buffered inputs present only one normalized TTL load, reducing fanout burden on upstream drivers.

Each decoder drives four active-low outputs (Y0–Y3) with totem-pole outputs-no open-collector or 3-state behavior. Propagation delays are symmetric (tPLH ≈ tPHL) and specified under CL = 50 pF, RL = 500 Ω, ensuring predictable timing in synchronous bus architectures.

Key Specifications

Pinout & Package

SOP-16 (NS) package: 10.4 mm × 5.3 mm × 2.0 mm max height, gull-wing leads, 1.27 mm pitch, pin 1 marked by beveled corner and quadrants Q1 orientation per tape reel specification.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar dual 2-to-4 decoder applications.

Compared with SN74LS139N and SN74HC139PWR, the SN74ALS139NSR delivers superior speed-power tradeoff for legacy 5 V TTL systems, with guaranteed 14 ns max delay and native compatibility - avoiding redesign effort or signal integrity compromises.

Availability

SN74ALS139NSR is available at Aetrix Electronics and suitable for memory address decoding, peripheral select logic, cascaded system decoding, and legacy bus interface applications requiring stable component supply across extended production lifecycles.

Supply support for SN74ALS139NSR 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 U.S.-based semiconductor company founded in 1930, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74ALS139NSR belongs to TI's advanced low-power Schottky (ALS) logic family, engineered for high-speed memory and data-routing systems where propagation delay and fanout efficiency are critical design constraints.

FAQ

What is the maximum propagation delay of the SN74ALS139NSR under recommended operating conditions?

The SN74ALS139NSR has a maximum propagation delay of 14 ns for both tPLH and tPHL when VCC = 4.5–5.5 V, CL = 50 pF, and TA = 0°C to 70°C. This value applies to transitions at A/B or G inputs to any Y output, as confirmed in the SDAS204A datasheet switching characteristics table. The SN74ALS139NSR achieves this performance using Schottky-clamped bipolar transistor design, making it suitable for sub-20 ns timing-critical decode paths.

Does the SN74ALS139NSR support 3-state or open-collector outputs?

No, the SN74ALS139NSR does not support 3-state or open-collector outputs. All eight outputs (1Y0–1Y3, 2Y0–2Y3) are active-pullup/pulldown totem-pole structures with defined HIGH and LOW states. The datasheet explicitly omits 3-state timing parameters (e.g., tPZL, tPLZ) and load circuit diagrams for 3-state operation - confirming standard push-pull drive. The SN74ALS139NSR is intended for wired-OR–free applications requiring deterministic logic levels.

Can the SN74ALS139NSR be used as a demultiplexer, and how is data routed?

Yes, the SN74ALS139NSR can operate as a dual 1-to-4 demultiplexer. The active-low enable inputs (1G/2G) serve as data inputs: when G is driven LOW, the selected Y output goes LOW per the A/B select code; all other outputs remain HIGH. For example, asserting 1G = LOW with 1A = LOW, 1B = HIGH drives 1Y2 LOW while 1Y0/1Y1/1Y3 stay HIGH - effectively routing a single control pulse to one of four destinations. This functionality is documented in the function table and logic diagram of the SN74ALS139NSR datasheet.

What is the input loading characteristic of the SN74ALS139NSR, and why does it matter?

The SN74ALS139NSR features fully buffered inputs, each presenting exactly one normalized TTL load (≈ 40 µA HI, ≈ 1.6 mA LO) to its driver - consistent across all 6 inputs (1G, 1A, 1B, 2G, 2A, 2B). This guarantees predictable fanout: a single 74ALS00 output can drive up to 20 SN74ALS139NSR inputs without exceeding current limits. The buffered design also isolates input nodes from internal logic switching noise, improving noise immunity in high-density layouts - a key advantage over unbuffered TTL families.

Is the SN74ALS139NSR pin-compatible with other 16-pin decoder packages like SN74ALS138 or SN74ALS154?

No, the SN74ALS139NSR is not pin-compatible with SN74ALS138 (3-to-8 decoder) or SN74ALS154 (4-to-16 decoder). Its SOP-16 pinout is unique to the dual 2-to-4 function: pins 1–7 and 9–15 are dedicated to two independent decoders with separate enables and outputs, unlike the single-enable, shared-input architectures of 138/154. Attempting physical substitution would result in incorrect logic behavior and potential bus contention. Always verify pin mapping against the NS package drawing (NS0016A) before PCB layout.