Texas Instruments SN74HCT138DE4

Overview

SN74HCT138DE4 from Texas Instruments is a 3-line to 8-line decoder/demultiplexer IC operating at 4.5 V–5.5 V, with 17 ns typical propagation delay (VCC = 5.5 V), ±4 mA output drive, TTL-compatible inputs, and 80 µA max supply current. It serves as a high-speed memory decoder or data router in address decoding paths for SRAM, EPROM, and FPGA configuration logic.

For engineers reviewing the SN74HCT138DE4 datasheet, SN74HCT138DE4 pinout, SN74HCT138DE4 application, or SN74HCT138DE4 equivalent, key selection criteria include enable-input architecture (two active-low + one active-high), output drive capability into LSTTL loads, propagation delay vs. memory access timing, and SOIC-16 package compatibility with industrial PCB layouts.

Technical Context

The SN74HCT138DE4 implements combinational logic decoding using three binary select inputs (A, B, C) and three enable terminals (G1, G2A, G2B) to activate exactly one of eight active-low outputs (Y0–Y7). Its function table defines mutually exclusive output states under valid enable conditions, with all outputs high when any enable is inactive.

Designed for memory subsystems, it minimizes system decoding delay by ensuring tpd (≤34 ns max at VCC = 5.5 V) remains below typical fast-memory access times. The enable structure supports cascading without external inverters-e.g., a 24-line decoder requires no added logic, while a 32-line implementation needs only one inverter.

Key Specifications

Pinout & Package

SN74HCT138DE4 is housed in a 16-pin SOIC (D) package measuring 9.90 mm × 3.90 mm with 1.27 mm lead pitch and gull-wing terminations. The package is RoHS-compliant, moisture sensitivity level 1, and rated for reflow up to 260 °C.

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 SN74HCT138DE4 uniquely balances TTL compatibility, industrial temperature range, and sub-35 ns delay-making it optimal for mixed-signal industrial controllers interfacing legacy peripherals without redesigning input drive stages.

Availability

SN74HCT138DE4 is available at Aetrix Electronics and suitable for memory address decoding, FPGA configuration routing, and industrial I/O expansion requiring stable component supply across extended product lifecycles.

Supply support for SN74HCT138DE4 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 with over 50 years of leadership in industrial and automotive electronics.

The SN74HCT138DE4 belongs to TI's HCT logic family, engineered specifically for high-speed memory decoding and data-routing applications where TTL compatibility, predictable propagation delay, and industrial temperature operation are essential.

FAQ

What is the maximum propagation delay of SN74HCT138DE4 at 5.5 V?

The maximum propagation delay (tpd) of SN74HCT138DE4 is 34 ns when VCC = 5.5 V and TA = 25 °C, measured from any select or enable input to any Y output under CL = 50 pF load. This value is specified in the Switching Characteristics table of the official TI datasheet SCLS171F.

Does SN74HCT138DE4 require external pull-up resistors on its outputs?

No, SN74HCT138DE4 does not require external pull-up resistors on its Y0–Y7 outputs because they are active-pull-down (totem-pole) outputs capable of sinking up to 4 mA. Pull-ups are only needed if interfacing with open-drain systems or implementing wired-OR logic, which is outside the device's intended use case.

Can SN74HCT138DE4 operate with a 3.3 V supply?

No, SN74HCT138DE4 is not rated for 3.3 V operation. Its absolute minimum VCC is 4.5 V, and input voltage thresholds (VIH = 2.0 V, VIL = 0.8 V) are defined only across 4.5–5.5 V. Using 3.3 V will result in undefined logic states and potential functional failure per TI's Recommended Operating Conditions.

How many enable inputs does SN74HCT138DE4 have, and what are their polarities?

SN74HCT138DE4 has three enable inputs: G1 (active-high), G2A (active-low), and G2B (active-low). All three must be simultaneously asserted (G1 = high, G2A = low, G2B = low) for any Y output to respond to A/B/C inputs; otherwise, all outputs remain high.

Is SN74HCT138DE4 pin-compatible with SN74LS138?

Yes, SN74HCT138DE4 is pin-compatible with SN74LS138 in the SOIC-16 package, sharing identical pin assignments for VCC, GND, A–C, G1–G2B, and Y0–Y7. However, SN74HCT138DE4 offers higher noise immunity, lower power, and CMOS-level output swing-making it a drop-in replacement where LS logic timing margins allow.