onsemi 74LCX646MSAX

Overview

74LCX646MSAX from Fairchild Semiconductor is a low-voltage octal transceiver/register with 5V-tolerant I/O, designed for bidirectional data flow between 2.5V/3.3V buses and 5V signal environments. It features registered bus transfer, real-time transparent mode, ±24 mA drive at 3.0V, 7.0 ns max propagation delay (VCC = 3.3V), and power-down high-impedance I/O. It is used in voltage-level bridging applications such as memory expansion interfaces in mixed-supply embedded systems.

For engineers reviewing the 74LCX646MSAX datasheet, pinout, applications, or equivalent options, key selection considerations include 5V tolerance on all I/O pins, dual-register storage capability (A/B), DIR/OE/SAB/SBA control logic, clocked register loading (CPAB/CPBA), and SSOP-24 package compatibility with space-constrained PCB layouts.

Technical Context

The 74LCX646MSAX implements a dual-register architecture with independent A and B data paths, each supporting both real-time (transparent) and stored (registered) transfer modes controlled by DIR, OE, SAB, and SBA inputs. Its operation relies on LOW-to-HIGH clock transitions (CPAB/CPBA) to latch data into either register, while output enable (OE) and direction (DIR) jointly determine active bus direction and output state.

It uses advanced CMOS process technology to achieve high-speed performance (150 MHz max clock frequency) with low static current (10 µA ICC max) and supports live insertion/withdrawal via power-down high-Z I/O states. All inputs and outputs are rated for 0–5.5 V DC, enabling safe interfacing across 2.5V, 3.3V, and 5V domains without external level shifters.

Key Specifications

Pinout & Package

74LCX646MSAX is housed in a 24-lead Shrink Small Outline Package (SSOP), EIAJ TYPE II, 5.3 mm wide (Package Number MSA24), with gull-wing leads and 0.65 mm pitch.

Key Features

Applications

Equivalent & Alternatives

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

Compared with 74LCX646MSAX, the 74ALVC164245DGGR provides higher channel count and true dual-supply flexibility but omits register-based data staging, while the SN74LVC8T245PW simplifies supply design and reduces footprint but sacrifices 5V tolerance at low VCC and eliminates synchronous storage capability.

Availability

74LCX646MSAX is available at Aetrix Electronics and suitable for memory expansion interfaces, processor bus bridging, hot-swappable module interfaces, and mixed-voltage backplane communication requiring stable component supply and long-term industrial availability.

Supply support for 74LCX646MSAX 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 and manufacturer of high-performance analog and mixed-signal ICs, acquired by ON Semiconductor in 2016; its legacy logic portfolio remains widely deployed in industrial and communications infrastructure.

The 74LCX646MSAX belongs to the LCX low-voltage CMOS logic family, engineered specifically for mixed-supply system interconnect-emphasizing 5V tolerance, low dynamic power, and high-speed register-controlled data transfer in space-constrained designs.

FAQ

What is the maximum clock frequency supported by the 74LCX646MSAX?

The 74LCX646MSAX supports a maximum clock frequency of 150 MHz under recommended operating conditions (VCC = 3.3V, CL = 50 pF, TA = −40°C to +85°C). This rating applies to CPAB and CPBA inputs and is validated across the full industrial temperature range. The 74LCX646MSAX achieves this performance while maintaining sub-7 ns propagation delay and low power consumption, making it suitable for high-throughput bus bridging applications.

Does the 74LCX646MSAX require external pull-up resistors on control pins?

Yes - to ensure high-impedance outputs during power-up or power-down, the OE pin must be tied to VCC through a pull-up resistor; the minimum value depends on the driver's current-sourcing capability, as specified in Note 1 of the datasheet. Other control inputs (DIR, SAB, SBA, CPAB, CPBA) should be actively driven or terminated to avoid floating states, per Recommended Operating Conditions.

Can the 74LCX646MSAX operate with a 2.5V supply and still interface with 5V signals?

Yes - the 74LCX646MSAX is fully 5V tolerant on all I/O pins (A0–A7, B0–B7) and control inputs (SAB, SBA, DIR, OE, CPAB, CPBA) when VCC is as low as 2.3V. At VCC = 2.5V, it maintains valid logic thresholds, 5.5V input tolerance, and ±8 mA output drive, enabling robust interoperability between 2.5V logic domains and 5V peripherals without level shifters.

What is the function of the SAB and SBA pins on the 74LCX646MSAX?

The SAB (Select A-to-B) and SBA (Select B-to-A) pins control multiplexing between real-time and registered data paths. When DIR = HIGH and OE = LOW, SAB determines whether Bus A drives Bus B directly (SAB = LOW) or whether the A register drives Bus B (SAB = HIGH). Similarly, SBA governs B-to-A transfer mode when DIR = LOW. These pins enable flexible bus management without additional logic gates.

Is the 74LCX646MSAX pin-compatible with other packages in the same family?

No - the 74LCX646MSAX (SSOP-24, MSA24) shares identical logic functionality and pin assignments with the SOIC (74LCX646WM, M24B) and TSSOP (74LCX646MTC, MTC24) variants, but mechanical dimensions, lead pitch, and thermal characteristics differ. PCB layout must be adapted per package; however, schematic symbol and net connectivity remain identical across all three packages of the 74LCX646MSAX family.