Texas Instruments SN74HCS138BQBR

Overview

SN74HCS138BQBR 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 control 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 three active-low strobe inputs (G₀, G₁) plus one standard strobe (G₂) for cascading and demux control.

For engineers reviewing the SN74HCS138BQBR datasheet, SN74HCS138BQBR pinout, SN74HCS138BQBR application, or SN74HCS138BQBR equivalent, key selection criteria include its Schmitt-trigger noise immunity, low ICC (100 nA typical), WQFN-16 package (3.5 mm × 2.5 mm), and guaranteed 16 ns max propagation delay at 6 V across full temperature range.

Technical Context

The SN74HCS138BQBR implements a single 3:8 decoder with three address inputs (A₀–A₂) and eight active-low outputs (Y₀–Y₇). Its functional mode is fully controlled by three strobe inputs: G₂ (active-high), G₁ and G₀ (both active-low); any active strobe forces all outputs high, enabling precise enable/disable sequencing in multi-device bus systems.

Schmitt-trigger inputs provide hysteresis (ΔV_T = 0.6 V min at 6 V), allowing reliable operation with slow-rising signals (<100 ns) and rejecting noise up to ±0.3 V peak-to-peak. Outputs use balanced CMOS push-pull drivers capable of sourcing/sinking 7.8 mA at 6 V while maintaining VOH ≥ 5.4 V and VOL ≤ 0.33 V under load.

Key Specifications

Pinout & Package

SN74HCS138BQBR uses a 16-pin WQFN package (3.50 mm × 2.50 mm, 0.5 mm pitch) with exposed thermal pad. The pad may be connected to GND or left floating - no connection to other signals or supplies is permitted.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HCS138D and SN74LV138A, SN74HCS138BQBR provides superior thermal performance in compact layouts, guaranteed Schmitt-trigger noise rejection for harsh environments, and broader voltage flexibility - making it the preferred choice for new designs targeting automotive, industrial, and high-reliability embedded systems.

Availability

SN74HCS138BQBR is available at Aetrix Electronics and suitable for industrial control systems, automotive body electronics, test equipment signal routing, and battery-powered sensor hubs requiring stable component supply across extended temperature ranges.

Supply support for SN74HCS138BQBR 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 personal electronics markets.

The SN74HCS138BQBR belongs to TI's HCS logic family, engineered for high-noise environments and extended temperature operation - specifically targeting industrial automation, automotive subsystems, and ruggedized instrumentation where reliability under electrical stress is critical.

FAQ

What is the maximum propagation delay of SN74HCS138BQBR at 6 V and 125°C?

The maximum propagation delay for SN74HCS138BQBR is 16 ns at 6 V across the full operating temperature range of –40°C to +125°C, as specified in the Switching Characteristics table. This value applies to both address-to-output (A₀–A₂ → Y₀–Y₇) and strobe-to-output (G₀/G₁/G₂ → Y₀–Y₇) paths. The SN74HCS138BQBR maintains this timing guarantee without derating, supporting deterministic timing in real-time control loops.

Can SN74HCS138BQBR operate with a 2.5-V supply in battery-powered applications?

Yes, SN74HCS138BQBR is fully specified from 2 V to 6 V, including 2.5 V. At 2.5 V, it delivers ±2.6-mA output drive, maintains 1.0-V input hysteresis, and draws only 0.1 µA typical supply current. This makes SN74HCS138BQBR suitable for ultra-low-power applications such as coin-cell–powered IoT edge nodes where the SN74HCS138BQBR decodes wake-up events without compromising battery life.

How should unused inputs be terminated on SN74HCS138BQBR?

All unused inputs on SN74HCS138BQBR must be tied to a valid logic level - either VCC or GND - to prevent floating states that cause increased ICC and potential oscillation. For example, if only two address lines are used, tie the third (e.g., A₂) to GND for consistent "0" addressing. Pull-up/pull-down resistors ≥10 kΩ are acceptable; direct connections are preferred for lowest leakage. The SN74HCS138BQBR's Schmitt-trigger inputs tolerate slow transitions but still require defined DC bias.

Does SN74HCS138BQBR support cascading with other decoders?

Yes, SN74HCS138BQBR supports hierarchical cascading via its three strobe inputs: G₀ and G₁ (active-low) and G₂ (active-high). In multi-stage decoding, outputs from a higher-order decoder can drive these strobes to enable subsets of SN74HCS138BQBR devices. For example, a master SN74HCS138BQBR's Y₀–Y₃ can enable four slave SN74HCS138BQBR units, expanding to 32 total outputs. All strobes are fully compatible with standard CMOS logic levels.

Is the thermal pad on SN74HCS138BQBR required to be soldered to ground?

No, the thermal pad on SN74HCS138BQBR may be connected to GND or left electrically floating - neither configuration is mandatory. TI specifies that the pad must not be connected to any signal or supply other than GND. When connected to GND, it improves thermal performance (R_θJB = 74.4°C/W); when floating, it retains full functionality but with slightly reduced heat dissipation. PCB layout guidelines recommend solder paste coverage ≥80% for mechanical reliability regardless of electrical connection.