Texas Instruments SN74LS240DBR

Overview

SN74LS240DBR from Texas Instruments is an octal inverting buffer/line driver with 3-state outputs, designed for memory address and bus driving applications. It features dual 4-bit channels, 15 ns maximum propagation delay at 5 V, ±24 mA output drive capability, and PNP input structure reducing DC loading on buses.

For engineers reviewing the SN74LS240DBR datasheet, SN74LS240DBR pinout, SN74LS240DBR application, or SN74LS240DBR equivalent, key selection criteria include 3-state bus interface capability, TTL-compatible logic levels, hysteresis-enhanced noise immunity, and SSOP-20 package suitability for high-density PCB layouts.

Technical Context

The SN74LS240DBR implements two independent 4-bit inverting buffer sections, each controlled by a dedicated active-low output-enable (G) input. Its PNP input stage minimizes input current draw, while built-in hysteresis provides 400 mV noise margin on all inputs.

It operates within 4.75–5.25 V supply range and supports bidirectional bus isolation via simultaneous control of both 4-bit groups. The device drives terminated lines down to 133 Ω and maintains stable 3-state behavior during power-up when G is pulled high.

Key Specifications

Pinout & Package

SN74LS240DBR is housed in a 20-pin SSOP (Shrink Small Outline Package) with 0.65 mm lead pitch and nominal body size 7.20 mm × 5.30 mm.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal inverting buffer applications.

Compared with SN74LS240N and SN74LS244DW, the SN74LS240DBR offers superior board-area efficiency in automated SMT lines, preserves required inversion for address decoding, and delivers identical bus-driving performance in a thermally optimized SSOP footprint.

Availability

SN74LS240DBR is available at Aetrix Electronics and suitable for memory subsystems, LED display controllers, network infrastructure equipment, and industrial I/O expanders requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS240DBR 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 company specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74LS240DBR belongs to TI's legacy LS-TTL logic family, engineered specifically for high-reliability bus buffering, memory address driving, and noise-critical industrial control interfaces.

FAQ

What is the function of the 1G and 2G pins on the SN74LS240DBR?

The 1G and 2G pins are active-low output-enable inputs that independently control the 3-state output drivers for Channel 1 (pins 1A1–1A4 → 1Y1–1Y4) and Channel 2 (pins 2A1–2A4 → 2Y1–2Y4). When either G pin is low, its associated four outputs pass inverted input signals; when high, those outputs enter high-impedance state. This enables selective bus arbitration in multi-device systems using the SN74LS240DBR.

Does the SN74LS240DBR support 3.3-V logic inputs?

Yes, the SN74LS240DBR inputs are tolerant down to 2 V and fully compatible with 3.3-V logic levels. Its VIH minimum is 2 V and VIL maximum is 0.8 V under recommended operating conditions, allowing direct connection to 3.3-V microcontrollers or FPGAs without level-shifting circuitry - a verified feature confirmed in TI's SDLS144D datasheet Section 6.3.

What is the maximum capacitive load the SN74LS240DBR can drive reliably?

The SN74LS240DBR is characterized for CL = 45 pF in switching tests (tPLH/tPHL), and its output structure supports stable operation into ≥50 pF loads with proper PCB layout. For heavier loads (>100 pF), propagation delay increases and edge integrity degrades; TI recommends limiting total node capacitance to 60 pF when operating at full speed, as validated in Figure 1 and Section 6.7 of the SN74LS240DBR datasheet.

Can the SN74LS240DBR be used in place of the SN74LS244 in a design?

No - the SN74LS240DBR is an inverting octal buffer, while the SN74LS244 is non-inverting. Swapping them without logic inversion compensation will cause functional failure in address/data paths. Electrical parameters (drive strength, delay, voltage thresholds) are closely matched, but polarity mismatch makes them functionally incompatible unless downstream logic accounts for inversion - a distinction explicitly defined in TI's function tables (Table 1 vs. Table 3).

What thermal considerations apply to the SN74LS240DBR in continuous operation?

The SN74LS240DBR in SSOP-20 has RθJA = 94.3°C/W. At full output loading (24 mA per pin × 8 pins), worst-case ICC ≈ 50 mA yields ~2.5 W dissipation, raising junction temperature by ~235°C above ambient - exceeding safe limits. Derating is essential: typical use limits to ≤4 outputs active simultaneously, keeping power below 0.5 W. Thermal relief pads and 2+ oz copper planes are recommended per TI's layout guidelines in Section 11.