Texas Instruments CY74FCT138CTSOC

Overview

CY74FCT138CTSOC from Texas Instruments is a 1-of-8 active-low decoder IC with three binary address inputs (A0–A2), three enable inputs (E1, E2 low-active; E3 high-active), and eight mutually exclusive active-low outputs (O0–O7). It operates at 5 V, delivers 64 mA sink / 32 mA source drive, supports partial-power-down via Ioff, and features edge-rate control for improved noise immunity in TTL-compatible systems.

For engineers reviewing the CY74FCT138CTSOC datasheet, CY74FCT138CTSOC pinout, CY74FCT138CTSOC application, or CY74FCT138CTSOC equivalent, key selection criteria include its 5 ns propagation delay (max), SOIC-16 package, Ioff-enabled power-down safety, matched rise/fall times, and compatibility with legacy TTL logic levels in memory address decoding and peripheral selection circuits.

Technical Context

The CY74FCT138CTSOC implements a dual 1-of-8 decoder architecture with three independent enable controls (E1, E2, E3) enabling cascaded expansion to 1-of-32 decoding using only four devices and one inverter. Its logic diagram confirms positive-logic enable gating feeding into a standard 3-to-8 binary decoder core.

It integrates edge-rate control circuitry to reduce switching noise and overshoot/undershoot, while Ioff circuitry actively disables outputs during power-down to prevent backflow current-critical for hot-swap and mixed-voltage system designs. Input thresholds (VIH = 2 V, VIL = 0.8 V) and output drive (VOH = 2.4 V min @ –32 mA, VOL = 0.55 V max @ 64 mA) are fully TTL-compatible.

Key Specifications

Pinout & Package

Package: SOIC-16 (DW), 10.3 mm × 7.5 mm body, 1.27 mm pitch, RoHS-compliant NiPdAu lead finish, MSL Level-1 (unlimited reflow).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 1-of-8 decoder applications.

Compared with CY74FCT138ATSOC, the CY74FCT138CTSOC offers tighter 5 ns timing for high-speed bus decoding; compared with SN74FCT138CTSO, it is a manufacturer-confirmed equivalent with identical parametric performance and qualification-making it suitable for drop-in use where TI's CY-prefix sourcing is preferred.

Availability

CY74FCT138CTSOC is available at Aetrix Electronics and suitable for industrial control systems, legacy bus interface modules, and memory-mapped peripheral selection requiring stable component supply across long production lifecycles.

Supply support for CY74FCT138CTSOC 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 U.S.-based semiconductor company founded in 1930, specializing in analog, embedded processing, and logic ICs with broad industrial, automotive, and communications portfolio coverage.

The CY74FCT138CTSOC belongs to TI's FCT logic family, designed specifically for high-drive, low-noise, TTL-compatible glue logic in industrial and military-grade systems requiring robust operation across extended temperature ranges.

FAQ

What is the maximum propagation delay of the CY74FCT138CTSOC?

The CY74FCT138CTSOC has a maximum propagation delay of 5 ns (from any address or enable input to any output) under recommended operating conditions (VCC = 4.75–5.25 V, TA = –40°C to +85°C). This value is specified for both tPLH and tPHL transitions and applies to all input-to-output paths including A→O, E1→O, E2→O, and E3→O.

Does the CY74FCT138CTSOC support partial-power-down operation?

Yes, the CY74FCT138CTSOC supports partial-power-down via its Ioff feature. When VCC = 0 V, the outputs are disabled and leakage current is limited to ±1 µA even with input/output voltages up to 4.5 V. This prevents damaging backflow current in mixed-power-domain or hot-swap systems-confirming safe operation during power sequencing.

Is the CY74FCT138CTSOC pin-compatible with older 74LS138 devices?

Yes, the CY74FCT138CTSOC is pin-compatible with the 74LS138 in SOIC-16 (DW) package. Pin assignments for A0–A2, E1–E3, O0–O7, VCC, and GND match exactly. However, the CY74FCT138CTSOC provides higher drive (64 mA sink), faster timing (5 ns vs 22 ns), and Ioff support-requiring no PCB changes but enabling performance upgrades.

What are the absolute maximum ratings for the CY74FCT138CTSOC?

The CY74FCT138CTSOC has absolute maximum ratings of –0.5 V to 7 V on all inputs and outputs, –0.5 V to 7 V on VCC, and –65°C to +135°C ambient operating temperature. Exceeding these values may cause permanent damage. Recommended operating conditions restrict VCC to 4.75–5.25 V and TA to –40°C to +85°C for guaranteed functionality.

Can the CY74FCT138CTSOC be used in cascaded 1-of-32 decoder configurations?

Yes, the CY74FCT138CTSOC supports cascaded 1-of-32 decoding using its three enable inputs. By connecting E1 and E2 to ground, driving E3 with a fourth address bit (A3), and feeding outputs to downstream CY74FCT138CTSOC devices, five address lines (A0–A4) can select one of 32 outputs. The datasheet explicitly validates this configuration with no additional components required.