onsemi 74LCX646WM

Overview

74LCX646WM from Fairchild Semiconductor is a low-voltage octal transceiver/register with registered bus interface, 5V-tolerant I/O, and dual-directional data flow control via DIR and OE pins. It operates at VCC = 2.3–3.6 V, delivers 7.0 ns max propagation delay at 3.3 V, supports ±24 mA output drive, and enables real-time or stored-mode bus transfer in industrial backplane and mixed-voltage system interconnects.

For engineers reviewing the 74LCX646WM datasheet, pinout, applications, or equivalent options, key selection criteria include 5V-tolerant I/O compatibility with legacy logic, registered vs. transparent mode operation, 24-pin SOIC package constraints, and OE/DIR-controlled directionality for bidirectional data isolation in voltage-translating bus systems.

Technical Context

The 74LCX646WM integrates eight D-type flip-flops per register (A and B), clocked by CPAB and CPBA to load data from either bus into internal storage. Its functional modes-real-time transfer, register-to-bus, and clocked storage-are selected via SAB/SBA and DIR under OE control.

It implements patented noise/EMI reduction circuitry and supports live insertion/withdrawal when OE is pulled up during power transitions. All inputs and outputs tolerate 0–5.5 V regardless of VCC (2.3–3.6 V), enabling safe interfacing between 2.5 V/3.3 V logic domains and 5 V peripherals without level shifters.

Key Specifications

Pinout & Package

74LCX646WM is housed in a 24-lead Small Outline Integrated Circuit (SOIC) package per JEDEC MS-013, 0.300" wide (Package Number M24B), with standard 1.27 mm pitch and gull-wing leads.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal transceiver/register applications.

Compared with 74LCX646WM, the 74ALVC164245DGGR offers wider voltage flexibility but no register capability, while the SN74LVC8T245PW provides simpler implementation at the expense of deterministic timing control and industrial-grade live-insertion assurance.

Availability

74LCX646WM is available at Aetrix Electronics and suitable for industrial backplane interconnect, mixed-voltage microcontroller bus extension, and telecom line card data multiplexing requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for 74LCX646WM 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

Fairchild Semiconductor was a leading designer of high-performance analog and mixed-signal semiconductors, acquired by ON Semiconductor in 2016; its legacy logic portfolio remains widely deployed in industrial and telecom infrastructure.

The 74LCX646WM belongs to the LCX low-voltage CMOS logic family, engineered specifically for mixed-voltage system interfacing with emphasis on 5 V tolerance, low-noise operation, and robust live-insertion behavior in mission-critical backplanes.

FAQ

What is the maximum clock frequency supported by the 74LCX646WM?

The 74LCX646WM supports a maximum clock frequency of 150 MHz under recommended operating conditions (VCC = 3.3 V, CL = 50 pF, TA = −40°C to +85°C). This rating is derived from AC electrical characteristics specifying tPHL/tPLH ≤ 7.0 ns at 3.3 V, enabling reliable operation in high-speed bus arbitration and data capture applications where precise timing alignment is required. The 74LCX646WM maintains this performance across its full industrial temperature range.

Does the 74LCX646WM require external pull-up resistors on any pins?

Yes - the 74LCX646WM requires a pull-up resistor on the OE pin to ensure high-impedance state during power-up or power-down transitions. Per Fairchild's Note 1, OE must be tied to VCC through a pull-up resistor whose minimum value depends on the current-sourcing capability of the driving source. This prevents bus contention during supply ramp-up and is essential for live insertion compliance. No other pins require mandatory external biasing.

Can the 74LCX646WM operate with a 2.5 V supply and still interface with 5 V signals?

Yes - the 74LCX646WM is explicitly rated for VCC = 2.3 V to 3.6 V and features 5 V-tolerant inputs and outputs. When powered at 2.5 V, it safely accepts and drives signals from 0 V to 5.5 V, making it suitable for interfacing 2.5 V logic domains with legacy 5 V peripherals without external level-shifting components. This capability is verified in the Absolute Maximum Ratings and Recommended Operating Conditions tables of the official datasheet.

What is the purpose of the SAB and SBA pins on the 74LCX646WM?

The SAB and SBA pins on the 74LCX646WM control multiplexing between real-time and registered data paths. SAB selects A-to-B operation: LOW enables transparent A→B transfer; HIGH enables Register A → B output. SBA selects B-to-A operation similarly. These pins work in conjunction with DIR and OE to configure four distinct bus-management functions - real-time transfer, register storage, and combined clocked-output modes - giving designers flexible control over data flow timing and buffering.

Is the 74LCX646WM pin-compatible with other members of the LCX family?

No - the 74LCX646WM is not pin-compatible with other LCX transceivers such as the 74LCX162245 or 74LCX16245 due to differences in pin assignment, register architecture, and control signal mapping. Its 24-pin SOIC layout is specific to the octal registered transceiver function, with dedicated CPAB/CPBA, SAB/SBA, and DIR/OE logic. Pin compatibility must be verified per device; substitution requires schematic and layout review. The 74LCX646WM's unique pinout is documented in the Connection Diagram of DS011997.