STMicroelectronics 74ACT138MTR

Overview

74ACT138MTR from STMicroelectronics is an advanced high-speed CMOS 3-to-8 line decoder (inverting) with TTL-compatible inputs, 5 ns typical propagation delay at 5 V, ±24 mA symmetrical output drive, and operation across 4.5–5.5 V supply. It features three enable inputs (G1, G2A, G2B) for cascading in memory address decoding systems and is pin- and function-compatible with standard 74-series 138 devices.

For engineers reviewing the 74ACT138MTR datasheet, 74ACT138MTR pinout, 74ACT138MTR application, or 74ACT138MTR equivalent, key selection criteria include its 50 Ω transmission-line driving capability, latch-up immunity, ESD tolerance (2 kV), balanced tPLH/tPHL delays, and guaranteed operation over –55°C to +125°C.

Technical Context

The device implements a fully decoded 3-bit binary input (A, B, C) to eight active-low outputs (Y0–Y7), with three independent enable controls enabling hierarchical address decoding. Its sub-micron C2MOS process ensures low ICC (4 µA max at 25°C) while maintaining TTL-level VIH (2.0 V min) and VIL (0.8 V max) compatibility.

Propagation delays are tightly balanced (tPLH ≅ tPHL), with worst-case values of 10.5 ns (A/B/C→Y) and 9.5 ns (G1→Y) across –55°C to +125°C. Output impedance is symmetrical (|IOH| = IOL = 24 mA min), supporting direct 50 Ω transmission-line termination without external resistors.

Key Specifications

Pinout & Package

TSSOP-16 package: 4.9–5.1 mm width, 6.25–6.5 mm length, 0.65 mm pitch, 0.5–0.7 mm lead length, surface-mount, lead-free compatible.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74ACT138DR and 74LVC138AD,118, the 74ACT138MTR uniquely delivers 5 V operation, TSSOP-16 compactness, and guaranteed 24 mA drive into 50 Ω loads - making it optimal for space-constrained, mixed-voltage industrial decoders requiring zero external termination.

Availability

74ACT138MTR is available at Aetrix Electronics and suitable for memory address decoding, bus arbitration logic, LED segment driver interfaces, and industrial I/O expansion requiring stable component supply across extended temperature ranges.

Supply support for 74ACT138MTR 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

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, designing and manufacturing analog, digital, and mixed-signal ICs for automotive, industrial, and consumer markets.

The 74ACT high-speed CMOS logic family was developed to bridge TTL performance with CMOS power efficiency - specifically targeting industrial control, instrumentation, and legacy-system upgrades requiring 5 V compatibility and robust operation.

FAQ

Is the 74ACT138MTR pin-compatible with older 74LS138 devices?

Yes - it is pin- and function-compatible with the 74LS138, including identical pin numbering and truth table behavior. However, it operates at higher speed (5 ns vs. 25 ns typical), draws far less quiescent current (4 µA vs. 19 mA), and provides stronger output drive (24 mA vs. 8 mA), requiring no design changes beyond verifying VCC stability and decoupling.

What is the maximum fan-out when driving 50 Ω transmission lines?

The device guarantees 24 mA sink/source per output into 50 Ω loads at VCC = 5 V, supporting direct connection to one 50 Ω line per output. For multi-drop or longer traces, use controlled-impedance layout and maintain CL ≤ 50 pF per output to preserve 5 ns timing integrity.

Does the 74ACT138MTR require external pull-up resistors on outputs?

No - its active-low outputs are designed for direct current sinking. Pull-ups are unnecessary unless interfacing with open-drain logic or specific voltage-level translation requirements. The 24 mA minimum sink strength allows direct LED or optocoupler drive without external components.

How does the enable logic differ between G1, G2A, and G2B?

G1 is active-high; G2A and G2B are active-low. All three enables must be simultaneously asserted (G1 = HIGH, G2A = LOW, G2B = LOW) for decoding to occur. This asymmetric enable structure simplifies cascading: G2A/G2B of a downstream device can be driven by a single Y output of an upstream device, while G1 remains tied to VCC.