Texas Instruments SN74HCS138DR

Overview

SN74HCS138DR from Texas Instruments is a 3-to-8 line decoder/demultiplexer with Schmitt-trigger inputs, designed for memory address decoding and data routing in industrial and automotive systems. It operates from 2 V to 6 V, delivers ±7.8 mA output drive at 6 V, supports –40°C to +125°C ambient temperature, and features low ICC (100 nA typical) for ultra-low-power applications.

For engineers reviewing the SN74HCS138DR datasheet, SN74HCS138DR pinout, SN74HCS138DR application, or SN74HCS138DR equivalent, key selection criteria include its triple strobe enable architecture (G2, G1̅, G0̅), active-low decoded outputs, Schmitt-trigger noise immunity, and compatibility with mixed-voltage logic interfaces across 2–6 V supply rails.

Technical Context

The SN74HCS138DR implements a high-speed silicon-gate CMOS 3:8 decoder with three address inputs (A2–A0) and three independent strobe inputs (G2, G1̅, G0̅) that collectively disable all outputs when asserted. Its Schmitt-trigger inputs provide hysteresis (ΔVT = 0.6–1.6 V depending on VCC), enabling robust operation with slow-rising signals and noise rejection up to ±1.6 V peak-to-peak.

Outputs are active-low, open-drain compatible in behavior (though push-pull), with VOH ≥ VCC – 0.1 V and VOL ≤ 0.33 V at full load. Propagation delay is as low as 7 ns (typical) at 6 V, and input capacitance is only 5 pF - supporting high-speed bus switching while minimizing dynamic current draw.

Key Specifications

Pinout & Package

SN74HCS138DR is supplied in SOIC-16 (D) package, measuring 9.90 mm × 3.90 mm, with standard 1.27 mm pitch and gull-wing leads suitable for automated assembly and reflow soldering.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138DR and CD74HCT138M96, the SN74HCS138DR uniquely combines Schmitt-trigger noise immunity, wide 2–6 V operation, and ultra-low quiescent current - making it optimal for ruggedized, battery-sensitive, or mixed-voltage industrial control designs where signal integrity cannot be assumed.

Availability

SN74HCS138DR is available at Aetrix Electronics and suitable for memory decoding, chip-select multiplexing, industrial I/O expansion, and automotive subsystem enable applications requiring stable component supply across extended temperature and voltage ranges.

Supply support for SN74HCS138DR 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 delivering analog and embedded processing solutions for industrial, automotive, and consumer applications.

The SN74HCS138DR belongs to TI's HCS logic family - engineered for high noise immunity, ultra-low power, and extended temperature operation in harsh real-world environments.

FAQ

What is the maximum capacitive load the SN74HCS138DR can drive while maintaining specified timing?

The SN74HCS138DR is characterized for CL = 50 pF in its switching specifications. At this load, propagation delay remains within datasheet limits (e.g., 7 ns typ at 6 V). Driving larger capacitances increases delay and transition time nonlinearly; for loads exceeding 50 pF, derating of timing margins and verification via board-level testing is required. The SN74HCS138DR datasheet explicitly recommends keeping total output capacitance ≤50 pF for guaranteed performance.

Can unused inputs on the SN74HCS138DR be left floating?

No - all unused inputs on the SN74HCS138DR must be terminated to either VCC or GND. Floating CMOS inputs cause undefined logic states, increased power consumption, and potential oscillation due to noise coupling. For the SN74HCS138DR, tie unused address or strobe inputs to VCC (for default high) or GND (for default low) using direct connections or 10-kΩ resistors, as specified in the device's Application and Implementation section.

Does the SN74HCS138DR support demultiplexing functionality?

Yes - the SN74HCS138DR supports demultiplexing by using one strobe input (e.g., G2) as the data input and A2–A0 as the channel select lines. When enabled, only the selected Yx output mirrors the G2 state (active-low), while all others remain high. This configuration is explicitly described in the SN74HCS138DR datasheet's Detailed Description and Typical Application sections.

What is the purpose of the thermal pad in the BQB package variant, and does SN74HCS138DR have one?

The thermal pad is exclusive to the WQFN (BQB) package variant and may be connected to GND or left floating - it is not present in the SOIC-16 (D) package used for SN74HCS138DR. Therefore, the SN74HCS138DR has no thermal pad; thermal management relies solely on standard SOIC-16 conduction paths through leads and PCB copper.

How does the SN74HCS138DR handle input signals slower than standard CMOS transitions?

The SN74HCS138DR handles slow inputs via integrated Schmitt-trigger inputs, which provide hysteresis (ΔVT = 0.6–1.6 V) to prevent metastability and chatter during gradual voltage transitions. Unlike standard CMOS inputs, the SN74HCS138DR does not require minimum input slew rate - enabling direct interface with RC-filtered signals, potentiometer wipers, or mechanical switch outputs without external debouncing circuitry.