Texas Instruments SN74HCS138QBQBRQ1

Overview

SN74HCS138QBQBRQ1 from Texas Instruments is an automotive-qualified 3-to-8 line decoder/demultiplexer with Schmitt-trigger inputs, operating from 2 V to 6 V supply, delivering ±7.8-mA output drive at 6 V, and featuring typical ICC of 100 nA for ultra-low-power operation in vehicle body control modules and infotainment system address decoding.

For engineers reviewing the SN74HCS138QBQBRQ1 datasheet, SN74HCS138QBQBRQ1 pinout, SN74HCS138QBQBRQ1 application, or SN74HCS138QBQBRQ1 equivalent, key selection criteria include AEC-Q100 Grade 1 qualification (–40°C to +125°C), wettable-flank WQFN package compatibility with automated optical inspection, Schmitt-trigger noise immunity, and three-strobe enable architecture for cascaded memory selection.

Technical Context

The SN74HCS138QBQBRQ1 implements a high-speed silicon-gate CMOS 3:8 decoder with three address inputs (A2–A0) and three independent strobe inputs (G2, G1, G0), where any active strobe forces all eight outputs high. Its Schmitt-trigger inputs provide hysteresis (ΔVT = 0.6 V min at 6 V) enabling robust operation with slow or noisy signals.

Outputs are active-low, open-drain compatible push-pull structures with balanced sourcing/sinking capability (±7.8 mA at 6 V), supporting demultiplexing by using a strobe as data input while selecting Y0–Y7 via address lines - all within a single 16-pin WQFN package with thermal pad.

Key Specifications

Pinout & Package

SN74HCS138QBQBRQ1 uses a 16-pin wettable-flank WQFN (WBQB) package (3.5 mm × 2.5 mm), optimized for AOI-compatible solder joint inspection and thermal performance (RθJA = 97.3°C/W). The exposed thermal pad may be connected to GND or left floating.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HCS138DQ1 and MC74HC138ADR2G, the SN74HCS138QBQBRQ1 uniquely combines AEC-Q100 Grade 1 qualification, wettable-flank WQFN packaging for AOI, and ultra-low ICC - making it the sole option qualified for volume production in modern automotive electronic control units requiring both reliability and miniaturization.

Availability

SN74HCS138QBQBRQ1 is available at Aetrix Electronics and suitable for automotive body control modules, infotainment system peripheral selection, ADAS camera multiplexing, and instrument cluster backlight sequencing requiring stable component supply across extended temperature and high-reliability manufacturing environments.

Supply support for SN74HCS138QBQBRQ1 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 company designing analog and embedded processing chips for industrial, automotive, and personal electronics markets.

The SN74HCS138QBQBRQ1 belongs to TI's HCS logic family - engineered specifically for automotive applications demanding AEC-Q100 qualification, low power, and noise-immune interface capability in space-constrained ECUs.

FAQ

What is the maximum junction temperature specification for SN74HCS138QBQBRQ1?

The absolute maximum junction temperature (TJ) for SN74HCS138QBQBRQ1 is 150°C, as defined in the Absolute Maximum Ratings table. This limit must not be exceeded during operation or transient conditions. At TA = 125°C and full load, the device's thermal resistance (RθJA = 97.3°C/W) ensures safe operation within this boundary when proper PCB copper area is used for heat spreading. Always verify actual TJ using measured ambient temperature and power dissipation in the final design.

Does SN74HCS138QBQBRQ1 support 1.8-V logic inputs?

No, SN74HCS138QBQBRQ1 does not support 1.8-V logic inputs. Its recommended operating supply range is 2 V to 6 V, and input voltage thresholds scale with VCC - minimum VT+ is 0.7 V at 2 V supply, which exceeds typical 1.8-V logic HIGH thresholds. Applying 1.8-V signals risks unreliable switching or increased dynamic current. For 1.8-V systems, consider level-shifting circuitry or a dedicated 1.8-V–tolerant decoder such as SN74LVC138A.

Can unused inputs on SN74HCS138QBQBRQ1 be left floating?

No, unused inputs on SN74HCS138QBQBRQ1 must not be left floating. Although Schmitt-trigger inputs reduce sensitivity to noise, floating inputs can drift into the undefined threshold region, causing excessive ICC, oscillation, or unpredictable output states. All unused address or strobe inputs must be tied to VCC or GND using direct connections or ≤10-kΩ pull-up/pull-down resistors - per TI's layout guidelines in Section 11.1 of the datasheet.

What is the purpose of the thermal pad on SN74HCS138QBQBRQ1's WBQB package?

The exposed thermal pad on SN74HCS138QBQBRQ1's WBQB package serves two primary functions: (1) lowering junction-to-board thermal resistance (RθJB = 66.4°C/W) to improve power dissipation during sustained output loading, and (2) enabling automated optical inspection (AOI) of solder fillets along the package sidewalls. TI specifies the pad may be connected to GND or left electrically unconnected - but it must be soldered to the PCB for mechanical stability and thermal performance.

How does the SN74HCS138QBQBRQ1 differ from standard 74HC138 devices in automotive use?

SN74HCS138QBQBRQ1 differs from standard 74HC138 devices in three critical ways for automotive use: (1) AEC-Q100 Grade 1 qualification (–40°C to +125°C), (2) enhanced ESD robustness (HBM ±4 kV, CDM ±1.5 kV), and (3) Schmitt-trigger inputs with guaranteed hysteresis (ΔVT ≥ 0.6 V at 6 V) for noise immunity. Standard 74HC138 lacks formal automotive qualification, has lower ESD ratings, and uses standard CMOS inputs - making SN74HCS138QBQBRQ1 the only version approved for production in safety-critical vehicle systems.