Texas Instruments SN74HCS138DYYR

Overview

SN74HCS138DYYR 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, supports –40°C to +125°C ambient temperature, delivers ±7.8-mA output drive at 6 V, and features typical supply current of 100 nA. Its three active-low strobes (G₀, G₁) and one standard strobe (G₂) enable precise output gating in shared-bus chip-select applications.

For engineers reviewing the SN74HCS138DYYR datasheet, SN74HCS138DYYR pinout, SN74HCS138DYYR application, or SN74HCS138DYYR equivalent, key selection criteria include Schmitt-trigger noise immunity, low ICC at wide VCC range, output drive capability at 6 V, thermal performance in SOT-23-THN package, and functional compatibility with cascaded decoder topologies.

Technical Context

The SN74HCS138DYYR implements a high-speed CMOS 3:8 decoder with balanced push-pull outputs and Schmitt-trigger input architecture. Its hysteresis (ΔVT = 0.6 V min at 6 V) enables reliable operation with slow-rising or noisy address/control signals, while its propagation delay (tpd = 7 ns typ at 6 V) ensures timing-critical demultiplexing in real-time control systems.

Strobe logic combines one standard (G₂) and two active-low (G₀, G₁) inputs to force all eight outputs high when any strobe is asserted - enabling hierarchical enable schemes across multiple decoders. Outputs are active-low, open-drain compatible only via external pull-ups, and functionally isolated during disable states to prevent bus contention.

Key Specifications

Pinout & Package

SOT-23-THN (16-pin) package with 4.20 mm × 2.00 mm body size and exposed thermal pad (not electrically connected; may be left floating or tied to GND).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138DR and 74LVC138AD,118, the SN74HCS138DYYR uniquely combines wide-voltage operation, Schmitt-trigger noise rejection, and ultra-low ICC - making it the only option qualified for harsh-environment industrial decoding where signal integrity and standby power are jointly constrained.

Availability

SN74HCS138DYYR is available at Aetrix Electronics and suitable for memory address decoding, chip-select multiplexing, industrial bus arbitration, and automotive subsystem enable applications requiring stable component supply across extended temperature and voltage ranges.

Supply support for SN74HCS138DYYR 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 SN74HCS138DYYR belongs to TI's HCS logic family, engineered for high-noise-immunity digital interfacing in mission-critical systems where reliability across voltage and temperature extremes is non-negotiable.

FAQ

What is the maximum capacitive load the SN74HCS138DYYR can drive while maintaining specified switching characteristics?

The SN74HCS138DYYR is characterized for CL = 50 pF in its switching specifications. Driving loads exceeding 50 pF increases propagation delay and transition time beyond datasheet limits - for example, tpd rises from 7 ns (typ) to >12 ns at 100 pF. To maintain timing compliance, keep total output capacitance ≤50 pF via short traces and avoid stubs or unterminated lines. The SN74HCS138DYYR datasheet explicitly defines this limit in Section 6.6.

Can unused inputs on the SN74HCS138DYYR be left floating, or must they be terminated?

Unused inputs on the SN74HCS138DYYR must never be left floating. Although Schmitt-trigger inputs tolerate slow transitions, floating nodes cause undefined logic states, increased ICC, and potential oscillation. TI mandates termination to VCC or GND - either directly or via 10-kΩ pull-up/down resistors - as stated in Section 9.2.1.2. Leaving A0, A1, A2, G₀, G₁, or G₂ unconnected violates the SN74HCS138DYYR absolute maximum ratings and risks functional failure.

Does the SN74HCS138DYYR support true open-drain operation, or are its outputs strictly push-pull?

The SN74HCS138DYYR has strictly push-pull CMOS outputs - not open-drain. Each Y0–Y7 output actively drives both HIGH (to VCC) and LOW (to GND). To achieve wired-AND behavior, external pull-up resistors must be added, and outputs must be disabled (via strobes) when not selected to prevent contention. This differs from open-drain decoders like SN74LV138A, and the SN74HCS138DYYR datasheet confirms push-pull topology in Section 8.3.1.

How does the thermal pad on the SN74HCS138DYYR package affect thermal performance, and how should it be connected?

The SN74HCS138DYYR's thermal pad is electrically isolated and may be left floating or connected to GND - but never to VCC or signal nets. TI specifies RθJA = 186.2°C/W with the pad unconnected; connecting it to a solid GND plane reduces junction-to-ambient resistance by ~15–20%. Layout guidelines in Section 11.2 require the pad to be soldered to a thermal land with ≥4 vias to inner GND layers. Improper pad handling directly impacts long-term reliability in +125°C operation.

Is the SN74HCS138DYYR pin-compatible with the SN74HC138 in the same SOT-23-THN package?

No - the SN74HCS138DYYR and SN74HC138 are not pin-compatible in SOT-23-THN. While both are 16-pin packages, the SN74HC138 is not offered in DYY (SOT-23-THN) packaging; its smallest variant is SOIC-16 (D) or TSSOP-16 (PW). The SN74HCS138DYYR is the only HCS-series 138 decoder in SOT-23-THN, and its pinout matches only other SN74HCS138 variants (D, PW, BQB), as confirmed in Table 5-1 and Section 5 of the SN74HCS138DYYR datasheet.