Texas Instruments SN74HCS138QWBQBRQ1

Overview

SN74HCS138QWBQBRQ1 from Texas Instruments is an automotive-qualified 3-to-8 line decoder/demultiplexer with Schmitt-trigger inputs, operating across 2 V to 6 V supply, delivering ±7.8-mA output drive at 6 V, and featuring AEC-Q100 Grade 1 qualification (–40°C to +125°C) for engine control, body electronics, and ADAS domain controllers.

For engineers reviewing the SN74HCS138QWBQBRQ1 datasheet, SN74HCS138QWBQBRQ1 pinout, SN74HCS138QWBQBRQ1 application, or SN74HCS138QWBQBRQ1 equivalent, key selection criteria include its wettable-flank WQFN-16 package, hysteresis-enabled noise immunity (ΔVT = 0.6 V min at 6 V), propagation delay of 7 ns typical at 6 V, and low ICC of 0.1 µA typical - all critical for robust automotive signal routing in noisy environments.

Technical Context

The SN74HCS138QWBQBRQ1 implements a single 3:8 decoder with three address inputs (A2–A0), one standard strobe (G2), and two active-low strobes (G1, G0); any active strobe forces all eight outputs high, enabling cascading and demultiplexing functions. Its CMOS push-pull outputs provide balanced sourcing/sinking capability.

Schmitt-trigger inputs deliver hysteresis (VT+ = 2.1 V, VT− = 1.2 V at 6 V), allowing reliable operation with slow-rising signals and rejecting noise up to 0.6 Vpp. The device operates in standard positive logic, with outputs normally high and only the selected output driven low when enabled.

Key Specifications

Pinout & Package

SN74HCS138QWBQBRQ1 uses a 16-pin wettable-flank WQFN (WBQB) package measuring 3.50 mm × 2.50 mm with exposed thermal pad. Wettable flanks enable AOI-compatible solder joint inspection for automotive production traceability.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar decoder/demultiplexer applications.

Compared with SN74HCS138DQ1 and SN74LV138AQPWRQ1, the SN74HCS138QWBQBRQ1 uniquely combines wettable-flank manufacturability, Schmitt-trigger noise immunity, and AEC-Q100 Grade 1 qualification in a thermally efficient 3.5 mm × 2.5 mm footprint - making it the only option qualified for safety-critical decoding in compact, high-reliability automotive modules.

Availability

SN74HCS138QWBQBRQ1 is available at Aetrix Electronics and suitable for engine control units, body control modules, ADAS camera interfaces, and electric power steering systems requiring stable component supply across extended automotive lifecycles.

Supply support for SN74HCS138QWBQBRQ1 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 designing analog, embedded processing, and connectivity solutions for industrial, automotive, and consumer markets.

The SN74HCS138QWBQBRQ1 belongs to TI's automotive-qualified HCS logic family, engineered specifically for noise-resilient signal routing in harsh-temperature vehicle subsystems where reliability and manufacturability are non-negotiable.

FAQ

What is the maximum capacitive load SN74HCS138QWBQBRQ1 can drive while maintaining specified timing?

The SN74HCS138QWBQBRQ1 is characterized for CL = 50 pF in its switching specifications. At this load, propagation delay remains ≤16 ns (max) at 6 V and 125°C. Driving larger capacitances increases tpd nonlinearly and may cause marginal timing in high-speed bus applications; TI recommends limiting total output capacitance to 50 pF for guaranteed performance across temperature and voltage extremes. The SN74HCS138QWBQBRQ1 datasheet specifies this condition explicitly in Section 6.6.

Does SN74HCS138QWBQBRQ1 require external pull-up or pull-down resistors on unused address inputs?

No - unused address inputs (A0–A2) and strobe inputs (G0–G2) on the SN74HCS138QWBQBRQ1 must be terminated to a defined logic level (VCC or GND), but external resistors are optional. Because the SN74HCS138QWBQBRQ1 has CMOS inputs with ±100 nA leakage, direct connection to VCC or GND is acceptable. A 10-kΩ resistor is recommended only if the input may float during power-up or partial system reset sequences to prevent metastability.

Can SN74HCS138QWBQBRQ1 outputs be paralleled for higher current drive?

Yes - two or more outputs of the SN74HCS138QWBQBRQ1 can be wired in parallel to increase sink/source capability, provided they are driven by identical address and strobe conditions. This technique is documented in TI's application notes for HCS logic and leverages the device's balanced CMOS push-pull structure. However, outputs must never be paralleled with opposing logic states, as that would create a short between VCC and GND.

Is the thermal pad of SN74HCS138QWBQBRQ1 required to be connected to ground?

Yes - the exposed thermal pad on the SN74HCS138QWBQBRQ1 must be soldered to a PCB ground plane. TI's mechanical drawings and thermal characterization data (RθJB = 66.4°C/W) assume the pad is connected to GND. Leaving it floating degrades thermal performance by >30% and risks exceeding junction temperature limits under sustained 7.8-mA output loading. The pad is electrically isolated from all signal pins and serves solely as a thermal and grounding path.

How does the Schmitt-trigger input of SN74HCS138QWBQBRQ1 improve noise immunity compared to standard CMOS inputs?

The SN74HCS138QWBQBRQ1 Schmitt-trigger inputs provide 0.6 V minimum hysteresis at 6 V supply (ΔVT = VT+ − VT−), meaning noise spikes must exceed this peak-to-peak amplitude to cause unintended state changes. Standard CMOS inputs have near-zero hysteresis and are vulnerable to oscillation when input signals cross the undefined threshold region. This makes the SN74HCS138QWBQBRQ1 robust against EMI from ignition systems, motors, or DC-DC converters - a key requirement validated under AEC-Q100 testing.