Texas Instruments SN74HCT138PW

Overview

SN74HCT138PW from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in TSSOP-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 memory address decoder or data routing switch in high-speed digital systems such as microcontroller peripheral expansion and FPGA I/O selection.

For engineers reviewing the SN74HCT138PW datasheet, SN74HCT138PW pinout, SN74HCT138PW application, or SN74HCT138PW equivalent, this page delivers verified functional modes, enable-input cascading logic, switching characteristics under 50-pF load, thermal resistance (108 °C/W), and real-world demultiplexing use cases with active-low G2A/G2B and active-high G1 enables.

Technical Context

The SN74HCT138PW 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 architecture supports hierarchical decoding via enable chaining - G1 can serve as data input for demultiplexing, while G2A and G2B allow direct cascade without external inverters.

It operates strictly within 4.5–5.5 V supply range with guaranteed VIH = 2.0 V and VIL = 0.8 V, ensuring robust noise margin against TTL-level signals. Propagation delay (tpd) is specified at 17 ns (typ) / 34 ns (max) at 5.5 V with CL = 50 pF, and output drive capability is ±4 mA at VOH/VOL limits, enabling direct interface with LSTTL loads.

Key Specifications

Pinout & Package

TSSOP-16 package (5.00 mm × 4.40 mm body, 1.2 mm max height), JEDEC MO-153 compliant, with gull-wing leads on 0.65-mm pitch. Pin 1 marked by index area; lead finish NiPdAu/Sn, 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 SN74HC138PW and 74ACT138SCX, the SN74HCT138PW uniquely balances TTL input compatibility, 5-V system integration, and sub-20 ns delay-making it the preferred choice for industrial controllers upgrading from 74LS138 without redesigning signal conditioning.

Availability

SN74HCT138PW is available at Aetrix Electronics and suitable for industrial control panels, microcontroller peripheral expansion, FPGA I/O management, and legacy bus interface designs requiring stable component supply across multi-year production cycles.

Supply support for SN74HCT138PW 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 50 years of innovation in industrial, automotive, and communications markets.

The SN74HCT138PW 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 critical.

FAQ

What is the maximum clock frequency supported by SN74HCT138PW?

The SN74HCT138PW is a combinational logic device with no internal clock; its effective operating frequency depends on input transition rate and system timing margins. With 17 ns typical propagation delay and 50-pF load, it supports reliable operation up to approximately 25 MHz in synchronous address decode paths, assuming clean edges and proper enable sequencing. The SN74HCT138PW does not specify fMAX but meets timing for standard 8051, Z80, and early ARM bus cycles.

Can SN74HCT138PW drive LEDs directly?

Yes, the SN74HCT138PW can drive LEDs directly when configured with active-low outputs: each Yx output sinks up to 4 mA at VOL ≤ 0.33 V (with IOL = 4 mA), sufficient for low-current indicator LEDs (e.g., 2-mA red LED with 1.8-V VF and 1.5-kΩ series resistor). Ensure total ICC remains within 80 µA quiescent limit during static display; avoid simultaneous multi-output activation to prevent exceeding absolute max output current (±25 mA per pin).

Is SN74HCT138PW pin-compatible with SN74LS138N?

Yes, the SN74HCT138PW is functionally and pin-compatible with SN74LS138N in decoder mode: identical pinout (TSSOP-16 matches PDIP-16 pin numbering), same enable logic (G1, G2A, G2B), and matching active-low Y0–Y7 outputs. However, SN74HCT138PW requires 4.5–5.5 V supply (vs LS138's 4.75–5.25 V), offers higher noise immunity, and eliminates LS-family DC fanout limitations - making it a direct drop-in replacement in most 5-V systems.

What is the purpose of the three enable inputs on SN74HCT138PW?

The three enable inputs (G1, G2A, G2B) provide flexible hierarchical control: G1 is active-high, G2A and G2B are active-low. All three must be asserted (G1=H, G2A=L, G2B=L) for decoding to occur. This allows daisy-chaining multiple SN74HCT138PW devices - e.g., using Y0–Y3 to enable lower-order decoders and Y4–Y7 for upper-order - eliminating external gating. G1 may also serve as a data line in demultiplexing mode.

Does SN74HCT138PW require external pull-up resistors on its outputs?

No, SN74HCT138PW outputs are push-pull and do not require external pull-ups. Each Yx output actively drives HIGH (VOH ≥ 3.7 V at IOH = –4 mA) or LOW (VOL ≤ 0.33 V at IOL = 4 mA) depending on decode state. Pull-ups would conflict with active-drive behavior and risk excessive current; they are only needed if interfacing with open-drain buses - which is not the intended use case for SN74HCT138PW.