STMicroelectronics 74AC138MTR

Overview

74AC138MTR from STMicroelectronics is an advanced high-speed CMOS 3-to-8 line decoder with inverting outputs, designed for memory address decoding and data routing in digital systems. It features propagation delay of 4.5 ns (typ. at V_CC = 5 V), symmetrical output drive (|I_OH| = I_OL = 24 mA min), and operates across 2 V to 6 V supply range. Its three enable inputs (G1, G2A, G2B) support cascaded decoding in multi-chip memory systems.

For engineers reviewing the 74AC138MTR datasheet, 74AC138MTR pinout, 74AC138MTR application, or 74AC138MTR equivalent, key selection criteria include guaranteed 50 Ω transmission-line driving capability, latch-up immunity, ESD tolerance (2 kV), balanced t_PLH/t_PHL, and compatibility with standard 74-series logic timing and pinout.

Technical Context

The 74AC138 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. All inputs are TTL-compatible and feature high noise immunity (V_NIH/V_NIL ≥ 28% V_CC).

It uses sub-micron silicon gate C2MOS technology, delivering low quiescent current (I_CC = 4 µA max at 25°C) while maintaining rail-to-rail output swing and symmetrical output impedance. Propagation delays are tightly matched (t_PLH ≅ t_PHL) across all signal paths including enable inputs.

Key Specifications

Pinout & Package

TSSOP-16 package (4.9 × 6.4 mm body, 0.65 mm pitch), thermally enhanced for high-density PCB layouts and reflow compatibility.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74AC138DR and 74LVC138ADTR2G, the 74AC138MTR uniquely balances 2–6 V operation, TSSOP thermal efficiency, and full 74-series pin/function compatibility-making it optimal for industrial upgrades and mixed-voltage legacy integration.

Availability

74AC138MTR is available at Aetrix Electronics and suitable for memory subsystems, industrial I/O expansion, legacy bus interface modules, and FPGA peripheral addressing requiring stable component supply across extended temperature ranges.

Supply support for 74AC138MTR 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, specializing in automotive, industrial, and power management ICs with broad analog, digital, and mixed-signal portfolios.

The 74AC logic family was engineered for high-speed, low-power TTL-compatible replacement in industrial control and instrumentation systems-prioritizing noise immunity, wide V_CC range, and robust ESD/latch-up performance.

FAQ

Is the 74AC138MTR pin-compatible with the 74LS138?

Yes-pinout, truth table, and enable logic (G1 active-low, G2A/G2B active-high) are identical. The 74AC138MTR improves propagation delay (4.5 ns vs. 25 ns), reduces I_CC (4 µA vs. 20 mA), and extends V_CC range (2–6 V vs. 4.75–5.25 V), making it a direct functional upgrade without PCB changes.

Can the 74AC138MTR drive 50 Ω transmission lines without series termination?

Yes-its guaranteed ±24 mA output drive at V_CC = 5 V meets the 50 Ω line requirement (V_OH ≥ 3.86 V at –24 mA, V_OL ≤ 0.5 V at +24 mA). This eliminates external series resistors in point-to-point routing, simplifying layout and preserving edge rate.

What is the maximum clock frequency supported for address decoding?

While not a clocked device, its 4.5 ns typical propagation delay supports reliable operation in systems with ≤20 MHz bus clocks (assuming 10 ns minimum address valid window). For 25 MHz+ systems, verify setup/hold timing with worst-case t_PD = 11.0 ns (–55°C to +125°C, V_CC = 5 V).

Does the 74AC138MTR require external pull-up resistors on outputs?

No-outputs are actively driven high or low per the truth table. Pull-ups would conflict with the internal totem-pole structure, increase power consumption, and risk contention. Outputs are designed for direct connection to CMOS/TTL inputs or 50 Ω loads.