Texas Instruments SN74AHCT138PWR

Overview

SN74AHCT138PWR from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in 16-pin TSSOP package, designed for high-speed memory decoding and data-routing applications. It features TTL-voltage-compatible inputs, three enable inputs (G1 active-high, G2A/G2B active-low), propagation delay as low as 1 ns at 5 V with 15 pF load, and operates across −40°C to +85°C.

For engineers reviewing the SN74AHCT138PWR datasheet, SN74AHCT138PWR pinout, SN74AHCT138PWR application, or SN74AHCT138PWR equivalent, key selection criteria include its 16-pin TSSOP footprint, 5.5 V absolute max supply rating, ±2000 V HBM ESD protection, and compatibility with legacy TTL logic levels in industrial control and embedded memory subsystems.

Technical Context

The SN74AHCT138PWR implements positive-logic 3-to-8 decoding with three independent enable controls that support cascading without external inverters - enabling 24-line decoding natively and 32-line decoding with one inverter. Its internal architecture routes binary-select inputs (A, B, C) to drive exactly one of eight active-low outputs (Y0–Y7) when all enables are asserted.

It uses advanced CMOS technology with TTL input thresholds (VIH = 2 V min, VIL = 0.8 V max), ensuring interoperability with 5 V TTL systems while maintaining low ICC (40 mA max at 5.5 V) and fast switching (tPLH/tPHL ≤ 12 ns at CL = 15 pF). The device meets JESD 17 latch-up immunity (>250 mA) and JEDEC JESD22 ESD standards.

Key Specifications

Pinout & Package

SN74AHCT138PWR is housed in a 16-pin Thin Shrink Small Outline Package (TSSOP-PW) with 0.65 mm lead pitch, 5.0 mm × 6.4 mm body size, and 1.2 mm max height. The package is RoHS-compliant, moisture-sensitive level 1 (260°C peak reflow), and optimized for automated SMT assembly.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HCT138PW and 74LV138PW, SN74AHCT138PWR delivers optimal balance of TTL compatibility, 12 ns speed, and 40 mA drive in a standard TSSOP footprint - making it the preferred choice for upgrading legacy 74LS138-based systems without redesigning interface logic or layout.

Availability

SN74AHCT138PWR is available at Aetrix Electronics and suitable for industrial control panels, embedded memory subsystems, and legacy-system upgrade programs requiring stable component supply and long-term obsolescence mitigation.

Supply support for SN74AHCT138PWR 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 over 90 years of innovation in industrial, automotive, and communications markets.

The SN74AHCT138PWR belongs to TI's 74AHCT logic family, engineered specifically for high-speed, TTL-compatible memory decoding and data routing in cost-sensitive, space-constrained embedded systems.

FAQ

What is the maximum supply voltage rating for SN74AHCT138PWR?

The absolute maximum supply voltage for SN74AHCT138PWR is 7 V, but the recommended operating range is 1.5 V to 5.5 V. Operating above 5.5 V risks violating recommended conditions and may degrade reliability or cause parametric shifts. All electrical characteristics - including propagation delay, output drive, and input thresholds - are specified only within the 1.5–5.5 V range per the official TI datasheet revision O (July 2024).

Does SN74AHCT138PWR support cascading with other decoders without external inverters?

Yes, SN74AHCT138PWR supports native cascading using its three enable inputs (G1 active-high, G2A/G2B active-low) to implement 24-line decoding without external inverters. For example, connecting G1 of a second SN74AHCT138PWR to Y7 of the first enables full 24-output expansion. A 32-line scheme requires only one external inverter due to the asymmetric enable structure - a capability explicitly documented in TI's functional description and application schematics.

What is the thermal resistance (RθJA) of SN74AHCT138PWR in its TSSOP package?

The junction-to-ambient thermal resistance (RθJA) for SN74AHCT138PWR in the PW (TSSOP-16) package is 135.9°C/W, as measured per JEDEC JESD51 standards. This value assumes standard 2-layer PCB layout with no thermal vias. Actual thermal performance improves significantly with copper pour on top/bottom layers and thermal vias under the exposed pad - though the PW package lacks an exposed thermal pad unlike RGY (VQFN).

Can SN74AHCT138PWR directly interface with 5 V TTL logic inputs?

Yes, SN74AHCT138PWR features TTL-voltage-compatible inputs with VIH(min) = 2.0 V and VIL(max) = 0.8 V at VCC = 4.5–5.5 V, matching standard 5 V TTL thresholds. This allows direct connection to 74LS, 74F, and other TTL families without level-shifting components. Input current remains within ±1 mA across the full input voltage range, ensuring clean logic transitions and predictable loading.

Is SN74AHCT138PWR pin-compatible with older 74LS138 or 74HC138 devices?

SN74AHCT138PWR is pin-compatible with 74LS138 and 74HC138 in the same 16-pin TSSOP (PW) package, sharing identical pin numbering, function mapping, and physical dimensions. However, differences in drive strength, propagation delay, and input thresholds mean direct substitution requires validation of timing margins and logic level compatibility - especially in mixed-technology systems where LS fanout or HC noise immunity assumptions may not hold.