Texas Instruments SN74ALS240A-1DW

Overview

SN74ALS240A-1DW from Texas Instruments is an octal noninverting buffer/driver with 3-state outputs, designed for memory address driving and bus interfacing in TTL-compatible systems. It features dual active-low output-enable inputs (1OE, 2OE), 48 mA low-level output current (IOL), SOIC-20 package, and operates from 4.5 V to 5.5 V at 0°C to 70°C.

For engineers reviewing the SN74ALS240A-1DW datasheet, SN74ALS240A-1DW pinout, SN74ALS240A-1DW application, or SN74ALS240A-1DW equivalent, key selection criteria include guaranteed 3-state leakage (±20 µA), propagation delay ≤9 ns (tPHL/tPLH), thermal resistance θJA = 58°C/W, and compatibility with ALS logic families in high-density bus architectures.

Technical Context

This device implements two independent 4-bit noninverting buffer sections, each controlled by a dedicated active-low output-enable input (1OE for Y1–Y4, 2OE for Y1'–Y4'). The pnp input structure reduces DC loading on preceding stages, enabling higher fan-in than standard TTL.

Its 3-state output architecture supports bidirectional bus control with fast enable/disable timing: tPZL ≤18 ns and tPHZ ≤10 ns under CL = 50 pF. The -1 suffix denotes enhanced drive capability-specifically, IOL = 48 mA (vs. 24 mA in standard SN74ALS240A)-valid only when VCC is 4.75–5.25 V.

Key Specifications

Pinout & Package

SN74ALS240A-1DW uses a 20-pin SOIC (DW) package, 7.5 mm × 12.83 mm body size, 1.27 mm lead pitch, and 2.65 mm max height per JEDEC MS-013. Pin 1 is located at the top-left corner with a beveled edge or index mark.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal 3-state buffer applications.

Compared with SN74ALS240ADW, SN74ALS240A-1DW delivers double the output sink strength for marginal bus loads without changing layout or timing constraints; versus SN74AS240ADW, it trades 3 ns of speed for significantly lower static power and relaxed input threshold compliance-ideal for cost-sensitive, thermally constrained legacy upgrades.

Availability

SN74ALS240A-1DW is available at Aetrix Electronics and suitable for memory address buffering, system bus interfacing, legacy system upgrades, and test equipment signal conditioning requiring stable component supply across extended production lifecycles.

Supply support for SN74ALS240A-1DW 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 founded in 1930, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

SN74ALS240A-1DW belongs to TI's legacy ALS (Advanced Low-Power Schottky) logic family, engineered for high-speed, low-power 5 V TTL-compatible systems in memory and bus-intensive applications from the 1980s onward.

FAQ

What is the maximum output sink current specification for SN74ALS240A-1DW?

The SN74ALS240A-1DW guarantees a low-level output current (IOL) of 48 mA per output, but only when VCC is maintained between 4.75 V and 5.25 V. This is the defining feature of the "-1" suffix variant, distinguishing it from the standard SN74ALS240A (24 mA). Exceeding this voltage range invalidates the 48 mA rating per the datasheet's footnote.

Does SN74ALS240A-1DW support true bidirectional data flow?

No, SN74ALS240A-1DW is a unidirectional noninverting buffer: inputs A1–A4 and A1'–A4' drive corresponding outputs Y1–Y4 and Y1'–Y4', respectively. While its 3-state outputs allow bus sharing, internal signal flow is strictly input-to-output; no built-in direction control or feedback paths exist. For bidirectional operation, external control logic or dedicated transceivers (e.g., SN74ALS245) are required.

What is the thermal resistance (θJA) of SN74ALS240A-1DW in its SOIC-20 package?

The junction-to-ambient thermal resistance (θJA) for SN74ALS240A-1DW in the SOIC (DW) package is 58°C/W, as specified in the Absolute Maximum Ratings table. This value assumes standard JEDEC 2-layer board conditions and is critical for calculating maximum allowable power dissipation (e.g., 1 W dissipation yields ~58°C rise above ambient) in thermally constrained designs.

Can SN74ALS240A-1DW be used as a direct replacement for SN74LS240 in existing designs?

Yes, SN74ALS240A-1DW is pin-compatible and functionally equivalent to SN74LS240, with identical SOIC-20 pinout and logic behavior. Key improvements include 2× output drive (48 mA vs. 24 mA), lower ICC (14–25 mA vs. ~30 mA), and faster propagation delay (≤9 ns vs. ≤15 ns). No PCB changes are needed, but verify VCC stability within 4.75–5.25 V to guarantee full 48 mA performance.

What are the recommended operating conditions for SN74ALS240A-1DW input voltage levels?

For SN74ALS240A-1DW, the recommended input voltage thresholds are VIL ≤ 0.8 V (low-level input) and VIH ≥ 2.0 V (high-level input) over the full operating temperature range (0°C to 70°C). These values ensure reliable switching margin against noise and process variation, and align with standard ALS logic family specifications-not LS or AS thresholds.