Texas Instruments SN74HC138NSR

Overview

SN74HC138NSR from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin SOP package, operating from 2 V to 6 V, with typical propagation delay of 15 ns at 6 V, ±4-mA output drive at 5 V, and active-low outputs enabling direct LED column sinking in matrix displays.

For engineers reviewing the SN74HC138NSR datasheet, SN74HC138NSR pinout, SN74HC138NSR application, or SN74HC138NSR equivalent, this page delivers verified functional modes, thermal metrics for NS package, enable-input logic interaction, and real-world use cases in memory decoding and LED scanning systems.

Technical Context

The SN74HC138NSR implements combinational 3-to-8 decoding with three binary select inputs (A, B, C) and three enable terminals (G1 high-active, G2A/G2B low-active), where all outputs remain high unless all enables are asserted and exactly one input combination is selected - yielding a single active-low output. Its CMOS architecture ensures rail-to-rail input compatibility and low static current.

Propagation delay varies with supply voltage (15 ns typ. at 6 V, 38 ns max. over temperature), and output transition times are specified under 50-pF load. The device supports cascading via enable pins without external inverters, enabling 24-line decoding natively and 32-line with one inverter.

Key Specifications

Pinout & Package

SOP (NS) package, 16-pin, body size 10.20 mm × 5.30 mm, RoHS-compliant, NIPDAU lead finish, MSL Level-1, tape-and-reel (2000 pcs).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138NSR, 74HC238PW offers inverted output polarity ideal for high-side drive, while SN74LV138APWR delivers superior speed and drive at 3.3 V but sacrifices 6-V operation - making SN74HC138NSR optimal for legacy 5-V or mixed-voltage industrial systems requiring proven reliability and wide supply tolerance.

Availability

SN74HC138NSR is available at Aetrix Electronics and suitable for industrial I/O expansion, LED matrix displays, and memory address decoding requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for SN74HC138NSR 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 ICs, with decades of experience in high-reliability industrial and automotive solutions.

The SN74HC138NSR belongs to TI's HC logic family, engineered for high-speed, low-power decoding in memory systems and data-routing applications where propagation delay and supply flexibility are critical.

FAQ

What is the maximum supply voltage rating for SN74HC138NSR?

The absolute maximum supply voltage for SN74HC138NSR is 7 V, but the recommended operating range is 2 V to 6 V. Operation above 6 V risks exceeding internal oxide breakdown limits and is not characterized for timing or drive performance. Always maintain VCC within 2–6 V for guaranteed functionality and lifetime reliability in the SN74HC138NSR.

Does SN74HC138NSR support cascading to decode more than eight outputs?

Yes, SN74HC138NSR supports native cascading using its three enable inputs: G1 (active-high), G2A and G2B (both active-low). By connecting outputs of one SN74HC138NSR to the enable pins of others, up to 24 lines can be decoded without external inverters; a 32-line decoder requires only one additional inverter. This is explicitly documented in the SN74HC138NSR functional description and timing tables.

What is the thermal resistance (RθJA) of the SN74HC138NSR in its SOP package?

The junction-to-ambient thermal resistance (RθJA) for SN74HC138NSR in the SOP (NS) package is 91.1 °C/W, measured under JEDEC JESD51-2A standard conditions. This value assumes a two-layer board with minimum copper; actual thermal performance improves significantly with internal ground/power planes and thermal vias - critical for sustained 4-mA output loading in enclosed industrial enclosures.

Can SN74HC138NSR drive LEDs directly without current-limiting resistors?

No - SN74HC138NSR cannot drive LEDs directly without external current-limiting resistors. While it provides ±4 mA output drive at 5 V, forward voltage and current requirements of LEDs vary widely. Omitting series resistors risks exceeding the SN74HC138NSR's absolute maximum output current (±25 mA) and may cause latch-up or accelerated wear. Always use per-LED resistors sized for target brightness and VCC.

How does the enable logic work on SN74HC138NSR?

SN74HC138NSR requires all three enables to be simultaneously asserted: G1 must be HIGH, and both G2A and G2B must be LOW. If any enable is violated, all eight outputs (Y0–Y7) go HIGH regardless of A/B/C inputs. This three-input enable scheme prevents partial decoding glitches and enables clean demultiplexing - for example, using G1 as a data input while holding G2A/G2B constant in SN74HC138NSR applications.