Texas Instruments SN74HCT138D

Overview

SN74HCT138D from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in SOIC-16 package, operating at 4.5–5.5 V, with 17 ns typical propagation delay, ±4-mA output drive, and TTL-compatible inputs. It serves as a high-speed memory address decoder in microcontroller-based embedded systems requiring precise enable-controlled channel selection.

For engineers reviewing the SN74HCT138D datasheet, SN74HCT138D pinout, SN74HCT138D application, or SN74HCT138D equivalent, key selection criteria include its three enable inputs (two active-low, one active-high) for cascading, low 80-µA max ICC quiescent current, and compatibility with LSTTL loads in industrial control logic and memory subsystems.

Technical Context

The SN74HCT138D implements combinational logic decoding using standard CMOS HCT process technology, ensuring TTL-level input thresholds while delivering CMOS power efficiency. Its functional mode is fully determined by binary-select inputs A/B/C and three independent enables (G1, G2A, G2B), enabling deterministic single-output activation per input combination.

Propagation delay is tightly specified across temperature and voltage: 17 ns typical at 5.5 V/25°C, degrading to 34 ns max at 4.5 V/25°C. Output drive capability remains stable at ±4 mA over full operating range (–40°C to +85°C), supporting direct interface to legacy LSTTL loads without level-shifting.

Key Specifications

Pinout & Package

SN74HCT138D is housed in a 16-pin SOIC (D) package measuring 9.90 mm × 3.90 mm, with standard 1.27-mm lead pitch and gull-wing leads. The package supports surface-mount reflow per JEDEC Level-1 rating (260°C peak).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138N and 74ACT138SCX, the SN74HCT138D uniquely balances TTL input compatibility, industrial temperature range, and LSTTL load drive-making it the only option that maintains drop-in functionality in legacy 5-V control systems without redesign.

Availability

SN74HCT138D is available at Aetrix Electronics and suitable for industrial PLC backplanes, memory subsystems, and legacy bus interface logic requiring stable component supply and long-term obsolescence management.

Supply support for SN74HCT138D 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 specializing in analog, embedded processing, and logic devices, with over 90 years of innovation in industrial, automotive, and communications markets.

The SN74HCT138D belongs to TI's 74HCT logic family, engineered specifically for high-speed memory decoding and data-routing applications where TTL compatibility, low power, and precise enable control are essential.

FAQ

What is the maximum propagation delay of SN74HCT138D under worst-case conditions?

The SN74HCT138D has a maximum propagation delay of 49 ns at 4.5 V and 25°C, and 54 ns at 4.5 V across the full industrial temperature range (–40°C to +85°C), as specified in the switching characteristics table. This value applies to transitions from any select or enable input to any Y output under CL = 50 pF loading. The SN74HCT138D maintains timing predictability even with marginal supply or temperature conditions, making it suitable for deterministic real-time decoding.

Can SN74HCT138D drive standard 74LS-series logic inputs directly?

Yes, the SN74HCT138D can drive up to 10 LSTTL loads directly due to its ±4-mA output drive capability at 5 V, which meets the 0.4-V VOL and 2.4-V VOH requirements of 74LS inputs. Its TTL-compatible input thresholds also allow seamless connection to 74LS outputs without level translation. This capability is explicitly confirmed in the device features and electrical characteristics sections of the TI datasheet for SN74HCT138D.

How does the enable architecture of SN74HCT138D support cascading for larger decoder configurations?

The SN74HCT138D uses three enables-G1 (active-high) and G2A/G2B (both active-low)-to simplify cascading. For example, two SN74HCT138D devices can implement a 4-to-16 decoder by connecting A/B/C to both, using the fourth address bit to control G1 on one device and G2A on the other. This eliminates external inverters, reduces board space, and ensures synchronized enable timing. The SN74HCT138D function table confirms this behavior across all valid input combinations.

Is SN74HCT138D suitable for use in demultiplexer mode, and how is that configured?

Yes, the SN74HCT138D operates as a 1-to-8 demultiplexer when G1 is used as the data input and A/B/C as the 3-bit channel address. With G2A and G2B held low, each rising edge on G1 propagates to exactly one Y output determined by A/B/C. This configuration is documented in the "Detailed Description" section of the SN74HCT138D datasheet and leverages the same internal logic as decoding-ensuring identical timing and drive performance in either mode.

What is the recommended bypass capacitor for SN74HCT138D and where should it be placed?

Texas Instruments recommends a 0.1-µF ceramic bypass capacitor placed as close as possible to the SN74HCT138D VCC and GND pins-ideally within 3 mm-to suppress high-frequency supply noise. The capacitor must be installed directly between VCC (pin 16) and GND (pin 7), with minimal trace length and no vias in the return path. This layout practice is specified in the "Power Supply Recommendations" and "Layout Guidelines" sections of the SN74HCT138D datasheet to ensure stable operation under fast switching transients.