Texas Instruments SN74LS138DRE4

Overview

SN74LS138DRE4 from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin SOIC package, operating from 4.75 V to 5.25 V with typical propagation delay of 22 ns (A→Y), fan-out of 20 TTL loads, and output drive capability of 24 mA sink / 0.4 mA source. It enables address decoding in microprocessor systems and memory selection in embedded controllers.

For engineers reviewing the SN74LS138DRE4 datasheet, SN74LS138DRE4 pinout, SN74LS138DRE4 application, or SN74LS138DRE4 equivalent, this page delivers verified electrical specs, validated pin functions, confirmed industrial-temperature-compatible variants, and two rigorously cross-checked alternative part options for legacy TTL system design and obsolescence mitigation.

Technical Context

The SN74LS138DRE4 implements three enable inputs (G1, G2A, G2B) with active-high and active-low logic to control output activation - all eight Y0–Y7 outputs are active-low and mutually exclusive when enabled. Its internal architecture uses standard TTL bipolar transistor logic with Schottky clamping to limit saturation and reduce propagation delay.

It operates strictly within the 0°C to +70°C commercial temperature range and requires a 5-V ±5% supply; no internal voltage regulation or level-shifting is provided. Output logic states are fully specified only under valid enable conditions and defined input voltage thresholds per LS-TTL standards (V_IL ≤ 0.8 V, V_IH ≥ 2.0 V).

Key Specifications

Pinout & Package

SN74LS138DRE4 is housed in a 16-pin SOIC (D) package measuring 10.4 mm × 5.4 mm × 2.0 mm max height, RoHS-compliant with NiPdAu lead finish, MSL Level-1 rating, and tape-and-reel packaging (2500 pcs/reel).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS138DR and SN74LS138N, the SN74LS138DRE4 offers identical core functionality and timing in a tube-packaged SOIC variant - making it optimal for low-to-mid volume production, engineering validation, or replacement in existing SOIC-based designs where reel logistics are unnecessary.

Availability

SN74LS138DRE4 is available at Aetrix Electronics and suitable for memory address decoding, peripheral selection in microcontroller systems, interrupt vector mapping, and LED display multiplexing requiring stable component supply across industrial control, test equipment, and legacy computing upgrades.

Supply support for SN74LS138DRE4 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 specializing in analog, embedded processing, and logic ICs with over 50 years of TTL product heritage.

The SN74LS138DRE4 belongs to TI's legacy 74LS logic family, designed specifically for reliable, low-cost address decoding and signal routing in commercial-grade digital systems operating at 5 V.

FAQ

What is the maximum supply voltage tolerance for SN74LS138DRE4?

The SN74LS138DRE4 is rated for 4.75 V to 5.25 V operation per TI's official datasheet. Exceeding 5.25 V risks permanent damage to internal bipolar junction transistors; operation below 4.75 V may cause marginal output drive or increased propagation delay. Always use a regulated 5-V supply with ≤±5% variation and local 0.1 µF ceramic decoupling at V_CC (pin 16) and GND (pin 8) for stable SN74LS138DRE4 performance.

Can SN74LS138DRE4 drive LEDs directly?

Yes, SN74LS138DRE4 can drive LEDs directly due to its 24 mA sink capability per output (Y0–Y7). For a typical red LED (2.0 V forward voltage, 10 mA target), use a 330 Ω series resistor between V_CC and the LED anode, connecting the cathode to an SN74LS138DRE4 output pin. Avoid sourcing current - outputs are active-low only, so LED-on state occurs when the output is low. Do not exceed 24 mA per pin or 50 mA total device sink current.

Is SN74LS138DRE4 pin-compatible with SN74LS138N?

Yes, SN74LS138DRE4 and SN74LS138N share identical pinouts and logic functions, but differ in package type: SN74LS138DRE4 uses SOIC-16 (surface-mount), while SN74LS138N uses PDIP-16 (through-hole). Their pin numbering and signal assignments (G1, G2A, G2B, A–C, Y0–Y7, V_CC, GND) match exactly. However, PCB layout and assembly processes differ - SOIC requires reflow soldering; PDIP supports hand-soldering or socket insertion.

What are the enable conditions required for SN74LS138DRE4 to produce valid outputs?

SN74LS138DRE4 produces valid active-low outputs (Y0–Y7) only when all three enables are simultaneously asserted: G1 = HIGH (≥2.0 V), G2A = LOW (≤0.8 V), and G2B = LOW (≤0.8 V). If any enable condition fails, all eight outputs go HIGH (open-collector equivalent state). This triple-enable structure prevents false decoding during reset, power-up, or bus contention - a key reliability feature confirmed in TI's SN74LS138 datasheet section 7.3.

Does SN74LS138DRE4 support mixed-voltage interfacing (e.g., 3.3 V logic inputs)?

No, SN74LS138DRE4 does not reliably accept 3.3 V logic inputs. As an LS-TTL device, it requires V_IH ≥ 2.0 V for HIGH recognition - while 3.3 V exceeds this threshold, its input structure lacks 3.3 V tolerance and may draw excessive current or exhibit timing uncertainty when driven by non-TTL sources. For 3.3 V systems, use a level-shifter or select a 74LVC138 instead. SN74LS138DRE4 inputs are designed exclusively for 5 V TTL-compatible signals.