Texas Instruments SN74HCT138PWT

Overview

SN74HCT138PWT from Texas Instruments is a 3-line to 8-line decoder/demultiplexer IC operating at 4.5–5.5 V, delivering 17 ns typical propagation delay (VCC = 5.5 V), ±4-mA output drive, and 80-µA max ICC. It features three enable inputs (two active-low, one active-high) for cascading in memory decoding and data routing systems.

For engineers reviewing the SN74HCT138PWT datasheet, SN74HCT138PWT pinout, SN74HCT138PWT application, or SN74HCT138PWT equivalent, this device serves as a TTL-compatible, high-speed address decoder in SRAM/ROM systems, FPGA I/O expansion, and bus demultiplexing where low power and deterministic timing are critical.

Technical Context

The SN74HCT138PWT implements a combinational logic decoder with binary-select inputs (A, B, C) and three enables (G1, G2A, G2B) that collectively determine which of eight active-low outputs (Y0–Y7) is asserted. Its HCT logic family ensures TTL-level input compatibility while maintaining CMOS power efficiency.

Propagation delay is tightly specified across voltage (4.5–5.5 V) and temperature (–40°C to +85°C), with switching characteristics measured under CL = 50 pF. The device supports demultiplexing via enable-as-data functionality and enables 24-line decoding without external inverters.

Key Specifications

Pinout & Package

TSSOP-16 package (PW), 5.00 mm × 4.40 mm body size, 1.2 mm max height, 0.65 mm lead pitch, RoHS-compliant NiPdAu/Sn lead finish, MSL Level-1.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138PWR and 74LVC138PW,118, the SN74HCT138PWT uniquely balances TTL input compatibility, 5-V system integration, 17-ns speed, and 80-µA quiescent current - making it optimal for legacy and industrial 5-V memory and control designs where signal integrity and timing predictability are non-negotiable.

Availability

SN74HCT138PWT is available at Aetrix Electronics and suitable for memory address decoding, FPGA I/O expansion, bus demultiplexing, and industrial control logic requiring stable component supply and long-term industrial lifecycle support.

Supply support for SN74HCT138PWT 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 solutions with over 90 years of innovation in industrial, automotive, and communications markets.

The SN74HCT138PWT belongs to TI's HCT logic family, engineered specifically for high-speed memory decoding and data-routing applications where TTL compatibility, low propagation delay, and industrial temperature operation are essential.

FAQ

What is the maximum propagation delay of SN74HCT138PWT at 5.5 V?

The maximum propagation delay (tpd) of SN74HCT138PWT is 49 ns at VCC = 5.5 V and TA = 25°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 conditions. The SN74HCT138PWT maintains consistent timing behavior across its full industrial temperature range (–40°C to +85°C), with worst-case tpd remaining within datasheet limits.

Can SN74HCT138PWT operate with a 3.3-V supply?

No, SN74HCT138PWT is not rated for 3.3-V operation. Its recommended operating voltage range is strictly 4.5 V to 5.5 V. At 3.3 V, the device may fail to meet VIH/VIL thresholds, exhibit increased propagation delay, or produce invalid outputs due to insufficient noise margins. For 3.3-V systems, consider SN74LVC138PW,118 or SN74AHC138PWR instead. The SN74HCT138PWT must be powered from a regulated 5-V rail to guarantee functional and timing compliance.

How many SN74HCT138PWT devices are needed to decode a 5-bit address to 32 outputs?

Two SN74HCT138PWT devices are required to implement a 5-bit to 32-line decoder. One device decodes the upper two bits (A4, A3) to generate four enable signals, each driving the G1 input of a separate SN74HCT138PWT configured for the lower three bits (A2–A0). This cascaded architecture uses only internal logic - no external inverters - and preserves the SN74HCT138PWT's 17-ns timing performance across all 32 outputs.

Does SN74HCT138PWT support demultiplexing functionality?

Yes, SN74HCT138PWT supports demultiplexing by using one of its enable inputs (typically G1) as the data input while holding the other enables active and applying address bits to A, B, and C. When G1 toggles, the selected Y output mirrors the G1 logic state - effectively routing a single input to one of eight outputs. This mode is explicitly supported in the device functional modes table and requires no external components. The SN74HCT138PWT maintains its full 17-ns propagation delay and ±4-mA drive strength in demultiplexer configuration.

What is the thermal resistance (RθJA) of SN74HCT138PWT in its TSSOP package?

The junction-to-ambient thermal resistance (RθJA) of SN74HCT138PWT in the TSSOP-16 (PW) package is 108°C/W, as specified in the Thermal Information table. This value assumes standard JEDEC High-K board conditions (2S2P). Under typical operating conditions with 80 µA ICC and 4 mA output loading, self-heating remains minimal (<1°C rise), confirming suitability for compact, thermally constrained PCB layouts without forced airflow or heatsinking. The SN74HCT138PWT's low power consumption directly contributes to its favorable thermal profile.