Texas Instruments SN74LS540N

Overview

SN74LS540N from Texas Instruments is an octal buffer/line driver with 3-state outputs, designed for bidirectional data bus interfacing in TTL-based digital systems. It features active-low output-enable (OE̅), 8-bit non-inverting logic, ±24 mA output drive capability, and operates over 0°C to 70°C. It is commonly used in microprocessor address/data bus isolation and memory I/O expansion.

For engineers reviewing the SN74LS540N datasheet, SN74LS540N pinout, SN74LS540N application, or SN74LS540N equivalent, this page delivers verified electrical parameters, package-specific mechanical data, functional role in bus-hold and load-driving contexts, and validated drop-in alternatives for legacy TTL system redesigns.

Technical Context

The SN74LS540N implements eight independent non-inverting buffers, each with a dedicated output-enable control input (OE̅) that places outputs in high-impedance state when asserted low. Its bipolar TTL process ensures compatibility with standard LS logic families and supports direct connection to 5 V CMOS inputs.

It provides symmetrical output drive: −24 mA sink and +15 mA source capability at V_CC = 5 V, enabling robust fan-out to up to 20 LS-TTL loads. Input thresholds are fixed at V_IL ≤ 0.8 V and V_IH ≥ 2.0 V, ensuring noise-immune switching in industrial control backplanes.

Key Specifications

Pinout & Package

SN74LS540N uses a 20-pin plastic dual in-line package (PDIP-N), 0.300″ wide, with standard through-hole mounting and 0.1″ pin pitch. Pin 1 is located at the left end of the top row, identified by a notch or dot marking.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS240N and SN74LS244N, the SN74LS540N offers unified 8-bit enable control and consistent non-inverting behavior-ideal for simple, single-channel bus buffering where minimal control logic and signal fidelity are prioritized over segmentation flexibility.

Availability

SN74LS540N is available at Aetrix Electronics and suitable for microprocessor bus interface, memory address latching, and industrial I/O expansion requiring stable component supply and long-term obsolescence mitigation.

Supply support for SN74LS540N 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 SN74LS540N belongs to TI's legacy 74LS logic family, engineered for reliability in commercial-grade digital systems requiring deterministic timing, robust noise immunity, and backward compatibility with decades-old designs.

FAQ

What is the function of pin 1 on the SN74LS540N?

Pin 1 on the SN74LS540N is the active-low output-enable (OE̅) input. When held low, it enables all eight non-inverting outputs (Y1–Y8) to drive their respective inputs (A1–A8); when high, all outputs enter high-impedance state. This pin is critical for bus-sharing applications and must be driven with TTL-compatible logic levels. The SN74LS540N relies on this single control line for synchronized 8-bit gating, distinguishing it from variants like SN74LS244N that use dual OE̅ pins.

Can SN74LS540N operate at 3.3 V?

No, the SN74LS540N is specified only for 4.75 V to 5.25 V operation and is not guaranteed to function correctly at 3.3 V. Its bipolar TTL process requires full 5 V supply to meet output voltage swing (V_OH ≥ 2.7 V, V_OL ≤ 0.5 V) and input threshold compliance. Using it at 3.3 V risks undefined logic states, excessive propagation delay, and potential damage due to internal biasing constraints. For 3.3 V systems, consider modern alternatives such as SN74LVC244A, not the SN74LS540N.

Is SN74LS540N RoHS compliant?

Yes, the SN74LS540N is RoHS compliant, featuring NiPdAu (NIPDAU) lead finish and meeting Directive 2011/65/EU requirements. Its PDIP package contains no lead in terminations, and TI confirms full compliance in the official packaging addendum. This applies specifically to the SN74LS540N variant-not older military or ceramic versions like SNJ54LS540J. Always verify lot-specific CoC documentation when sourcing SN74LS540N for regulated production.

How does SN74LS540N differ from SN74LS541N?

The SN74LS540N is a non-inverting octal buffer, while the SN74LS541N is its inverting counterpart (Y = A̅). Both share identical pinout, DC/AC specifications, and 3-state architecture, but differ exclusively in logic polarity. This means SN74LS540N and SN74LS541N are not functionally interchangeable without adjusting upstream/downstream logic. Circuit redesign is required if substituting one for the other, as the SN74LS540N preserves input signal phase whereas SN74LS541N inverts it.

What is the maximum capacitive load SN74LS540N can drive reliably?

The SN74LS540N is characterized to drive up to 45 pF load capacitance while maintaining specified propagation delay (15 ns max) and output transition times. Exceeding this limit increases rise/fall times and may cause timing violations in high-speed bus applications. For heavier loads (e.g., >60 pF), derating is required: reduce clock frequency, add series termination, or cascade with a second SN74LS540N stage. TI's SDLS180 datasheet confirms this 45 pF rating under standard test conditions for the SN74LS540N.