Texas Instruments CD74AC138BQBR

Overview

CD74AC138BQBR from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin WQFN package, operating from 1.5 V to 5.5 V with ±24 mA output drive, balanced propagation delays, and SCR-latchup-resistant CMOS process. It serves as a high-speed memory decoder in industrial control systems requiring low-noise, fast address decoding.

For engineers reviewing the CD74AC138BQBR datasheet, CD74AC138BQBR pinout, CD74AC138BQBR application, or CD74AC138BQBR equivalent, key selection criteria include supply voltage range (1.5–5.5 V), output drive capability (±24 mA), propagation delay at 5 V (2.8–11 ns), enable-input architecture (G1, G2A, G2B), and thermal pad–enabled WQFN-16 package for high-density PCB layouts.

Technical Context

The CD74AC138BQBR implements positive-logic 3-to-8 decoding with three active-low enable inputs (G2A, G2B) and one active-high enable input (G1), requiring all enables to be asserted for output activation. Its function table defines eight mutually exclusive active-low outputs (Y0–Y7) based on binary-select inputs A, B, C.

It uses Schmitt-trigger inputs for noise immunity and features balanced tPLH/tPHL delays across all input-to-output paths. The device supports cascading via enable inputs and maintains stable operation across –55°C to 125°C ambient temperature with ESD protection exceeding ±2 kV per MIL-STD-883 Method 3015.

Key Specifications

Pinout & Package

The CD74AC138BQBR is housed in a 3.5 mm × 2.5 mm, 0.5 mm pitch, 16-pin WQFN (BQB) package with an exposed thermal pad. This thermally enhanced package requires soldering the pad to a PCB copper pour for optimal junction-to-board thermal resistance (RθJB = 12.5°C/W typical).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74AC138PWR and CD74HC138E, the CD74AC138BQBR delivers superior thermal performance via its exposed pad, supports 1.5 V operation for modern low-voltage MCUs, and provides 6× higher output drive-making it the preferred choice for compact, high-reliability industrial controllers.

Availability

CD74AC138BQBR is available at Aetrix Electronics and suitable for industrial automation, embedded instrumentation, and aerospace avionics requiring stable component supply, extended-temperature operation, and RoHS-compliant packaging.

Supply support for CD74AC138BQBR 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 decades of heritage in high-reliability logic families.

The CD74AC138BQBR belongs to TI's AC-series advanced CMOS logic line, engineered for high-speed memory decoding and data-routing applications demanding low propagation delay, wide supply range, and robust noise immunity.

FAQ

What is the minimum supply voltage for reliable operation of the CD74AC138BQBR?

The CD74AC138BQBR operates reliably down to 1.5 V, as specified in the Recommended Operating Conditions table. At this voltage, it maintains defined logic thresholds (VIH = 1.2 V, VIL = 0.3 V) and functional timing-enabling direct use with 1.8 V microcontrollers and low-power sensor nodes. Performance validation at 1.5 V includes propagation delay (138 ns max) and output drive (±24 mA) across the full temperature range.

Does the CD74AC138BQBR require external pull-up resistors on its outputs?

No, the CD74AC138BQBR does not require external pull-up resistors. Its outputs are actively driven high or low when enabled, and enter high-impedance (Hi-Z) state when disabled-eliminating static current draw and ensuring clean bus release. Pull-ups would conflict with active-drive behavior and risk excessive current during output transitions.

How should the thermal pad on the CD74AC138BQBR WQFN package be handled in PCB layout?

The exposed thermal pad on the CD74AC138BQBR must be soldered to a solid copper pour connected to the system ground plane. TI recommends ≥85% solder paste coverage, 0.125 mm stencil thickness, and at least four thermal vias (0.2 mm diameter) beneath the pad, filled or tented to prevent solder wicking. Omitting this compromises thermal resistance (RθJA increases from 83.9°C/W to >120°C/W) and risks long-term parametric drift.

Can the CD74AC138BQBR be used in cascaded decoder configurations?

Yes-the CD74AC138BQBR supports cascading via its three enable inputs (G1, G2A, G2B). For example, one device can decode higher-order address bits to enable the G1 inputs of multiple downstream CD74AC138BQBR units, each handling lower-order bits. This architecture scales to 24-bit or 32-bit address spaces, as shown in TI's Application Note Figure 7-1 and Figure 7-2.

What is the maximum clock frequency supported by the CD74AC138BQBR for address decoding?

The CD74AC138BQBR does not operate on a clock-it is combinational logic with propagation delay as the key timing parameter. At 5 V, its worst-case tPLH/tPHL is 11 ns, supporting address bus toggle rates up to ~45 MHz (1/(2 × 11 ns)). System-level maximum frequency depends on setup/hold margins relative to the controlling clock edge, not internal oscillation.