Texas Instruments CD74HC138MT

Overview

CD74HC138MT from Texas Instruments is a high-speed CMOS 3-to-8 line decoder/demultiplexer with active-low outputs, operating from 2 V to 6 V, featuring 13 ns typical propagation delay at 5 V/15 pF, -55°C to +125°C temperature range, and three enable inputs (G2, G1, G0) for cascading in memory address decoding or I/O port selection applications.

For engineers reviewing the CD74HC138MT datasheet, CD74HC138MT pinout, CD74HC138MT application, or CD74HC138MT equivalent, key selection criteria include its inverting output logic, wide supply voltage tolerance, industrial temperature rating, low power consumption versus LSTTL, and compatibility with standard CMOS and TTL-level inputs.

Technical Context

The CD74HC138MT implements a 3-input binary address decoder with three independent enable inputs-G2 (active-high), G1 and G0 (both active-low)-that collectively gate all eight Y0–Y7 outputs into a high-impedance or forced-high state when asserted. Its logic function strictly follows the '138 truth table where only one output goes low per valid address combination when all enables are satisfied.

It uses silicon-gate CMOS technology with balanced rise/fall times, fanout of 10 LSTTL loads on standard outputs, and input clamp diode protection rated for ±20 mA. Propagation delay and transition time are characterized across full temperature and voltage ranges, with dynamic power dissipation modeled via Cpd = 67 pF at 5 V.

Key Specifications

Pinout & Package

CD74HC138MT is supplied in a 16-pin SOIC (D) package measuring 9.90 mm × 3.90 mm, with standard surface-mount footprint and gull-wing leads.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138DR and MC74HC138ADTR2G, CD74HC138MT offers proven long-term stability in extended temperature environments and legacy design continuity, while trading minor delays and power for field-proven robustness in mission-critical industrial deployments.

Availability

CD74HC138MT is available at Aetrix Electronics and suitable for memory address decoding, I/O port selection, LED display multiplexing, and industrial control logic requiring stable component supply across extended temperature ranges.

Supply support for CD74HC138MT 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 headquartered in Dallas, Texas, delivering analog and embedded processing solutions for industrial, automotive, personal electronics, and communications markets.

CD74HC138MT belongs to TI's CD74HC logic family, engineered for high-speed, low-power CMOS operation in harsh environments - specifically targeting industrial automation, test equipment, and avionics where reliability across -55°C to +125°C is non-negotiable.

FAQ

What is the maximum clock/data rate supported by CD74HC138MT in demultiplexer mode?

CD74HC138MT does not operate as a clocked device but as combinational logic. Its maximum usable data rate in demultiplexer mode is governed by propagation delay (13 ns typ) and output transition time (15 ns typ), supporting reliable switching up to approximately 20 MHz for clean digital waveforms with proper load capacitance (≤50 pF). For sustained high-frequency use, ensure VCC bypassing with a 0.1 µF capacitor near pin 16 and verify setup/hold margins in your specific timing path. The CD74HC138MT itself imposes no internal clock limit.

Can CD74HC138MT interface directly with 3.3 V microcontrollers?

Yes, CD74HC138MT supports 2 V to 6 V operation and has TTL-compatible input thresholds: VIH = 3.15 V min and VIL = 1.35 V max at VCC = 4.5 V. When powered at 3.3 V, its VIH is ~2.31 V (30% of 3.3 V), which safely accepts 3.3 V logic highs from standard microcontrollers. Its outputs swing rail-to-rail, providing 3.3 V compatible logic levels. No level-shifting circuitry is needed for direct connection to 3.3 V GPIOs driving CD74HC138MT inputs.

Does CD74HC138MT support cascading with other decoders, and how is it implemented?

Yes, CD74HC138MT supports cascading via its three enable inputs: G2 (active-high), G1 and G0 (both active-low). To cascade two devices, connect address lines A2–A0 in parallel, use higher-order address bits to control enables (e.g., A3 drives G2 of first device and G0 of second), and wire outputs to disjoint memory regions. All enables must be satisfied simultaneously for output activation. This architecture allows expansion to 1-of-16, 1-of-24, or larger decoding trees without external gates - a core design feature confirmed in the CD74HC138MT function tables and application notes.

What is the output drive strength of CD74HC138MT, and can it drive LEDs directly?

CD74HC138MT provides ±4 mA output current per Y0–Y7 pin at VCC = 4.5 V, sufficient to drive standard 2 mA LEDs in common-cathode configurations (with current-limiting resistor). For example, with VCC = 5 V and red LED VF ≈ 1.8 V, a 1 kΩ resistor yields ~3.2 mA sink current - well within spec. However, continuous DC drive of multiple outputs may exceed total package current limits (±50 mA), so duty cycling or external buffering is advised for high-brightness or multi-digit LED applications. Always reference the CD74HC138MT absolute maximum ratings before direct LED drive.

Is CD74HC138MT RoHS compliant, and what is its moisture sensitivity level?

CD74HC138MT is RoHS compliant per TI's packaging addendum, with lead finish NIPDAU (nickel-palladium-gold). Its moisture sensitivity level (MSL) is Level-1 at 260°C peak reflow, meaning it is not moisture-sensitive and requires no baking prior to soldering. This MSL rating applies to the SOIC (D) package variant and is documented in TI's official orderable addendum. As CD74HC138MT is marked "Obsolete" in TI's current status, Aetrix Electronics maintains traceable, tested inventory meeting original TI specifications including RoHS and MSL compliance.