Texas Instruments SN74HC251D

Overview

SN74HC251D from Texas Instruments is an 8-to-1 data selector/multiplexer with complementary 3-state outputs, designed for bus-oriented digital systems. It features full binary decoding of three select lines (A, B, C), strobe-controlled output enable (OE), and operates across 2 V to 6 V supply voltage. Its ±6-mA output drive at 5 V and typical propagation delay of 9 ns support reliable parallel-to-serial conversion in industrial control logic.

For engineers reviewing the SN74HC251D datasheet, SN74HC251D pinout, SN74HC251D application, or SN74HC251D equivalent, this page delivers verified electrical specs, functional mode behavior, package-specific thermal metrics, and real-world multiplexer integration guidance - all grounded in TI's SCLS132F production data sheet (Rev. F, Feb 2022).

Technical Context

The SN74HC251D implements a single 8-input multiplexer with dual complementary outputs (Y and W), where Y provides true data and W provides inverted data. All eight data inputs (D0–D7) are routed through a binary decoder controlled by A/B/C select lines, while OE enables or disables both outputs simultaneously into high-impedance state.

Its CMOS architecture ensures low static current (≤80 μA ICC), input leakage ≤1 μA, and compatibility with LSTTL loads (up to 15). Switching performance is characterized at CL = 50 pF and CL = 150 pF, with tpd as low as 19 ns at VCC = 6 V and TA = 25°C.

Key Specifications

Pinout & Package

SN74HC251D is housed in a 16-pin SOIC (Small Outline Integrated Circuit) package with nominal body size 9.90 mm × 3.90 mm and 1.27-mm lead pitch. The package is RoHS-compliant, moisture sensitivity level (MSL) 1, and rated for reflow up to 260°C.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 8-to-1 multiplexer applications.

Compared with SN74HC151N and 74LVX151M, the SN74HC251D uniquely balances wide supply range (2–6 V), SOIC packaging, and proven 3-state bus compatibility - making it the preferred choice for industrial control and education platforms requiring robustness and layout flexibility.

Availability

SN74HC251D is available at Aetrix Electronics and suitable for industrial PLC I/O expansion, automated test equipment signal routing, embedded data acquisition front-ends, and digital logic education platforms requiring stable component supply and long-term manufacturability.

Supply support for SN74HC251D 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, embedded processing, and connectivity solutions with emphasis on reliability, longevity, and broad technical documentation support.

The SN74HC251D belongs to TI's 74HC logic family, engineered for high-noise-immunity, low-power CMOS operation in commercial and industrial control systems - emphasizing interoperability, bus compatibility, and ease of integration into legacy and new designs.

FAQ

What is the function of the W output on the SN74HC251D?

The W output on the SN74HC251D is the logical complement of the Y output. When a specific data input (e.g., D3) is selected via A/B/C, Y reflects D3's logic level while W reflects its inverse. Both outputs are simultaneously enabled or disabled by OE, allowing true/inverted signal pairs without external inverters - a key feature confirmed in the function table and block diagram of the SN74HC251D datasheet (SCLS132F, Section 7.3).

Can the SN74HC251D operate reliably at 3.3 V?

Yes, the SN74HC251D operates reliably across 2 V to 6 V, including 3.3 V. At VCC = 3.3 V, its VIH is guaranteed ≥2.31 V and VIL ≤1.09 V per recommended operating conditions (Section 5.2), and typical tpd is ~25 ns with CL = 50 pF. This makes SN74HC251D fully compatible with 3.3-V microcontrollers and FPGAs while retaining noise margin and drive strength.

Is the SN74HC251D pin-compatible with the SN74HC151?

Yes, the SN74HC251D is functionally and pin-compatible with the SN74HC151 in SOIC (D), SSOP (DB), PDIP (N), and other shared packages. Both share identical pin assignments for D0–D7, A/B/C, Y, W, OE, VCC, and GND. The SN74HC251D is explicitly described as the "3-state version of 'HC151" in TI's official features list, confirming direct replacement capability in bus-interface applications.

What is the maximum capacitive load the SN74HC251D can drive?

The SN74HC251D is characterized for CL = 50 pF and CL = 150 pF in its switching specifications (Sections 5.5 and 5.6). While not specified beyond 150 pF, its ±6-mA output drive at 5 V supports heavier loads than standard LS-TTL. For reliable timing, keep total net capacitance ≤150 pF; for higher loads, add a buffer stage. This limit is derived from measured tpd degradation in the SN74HC251D datasheet (SCLS132F, Table 5-5).

Does the SN74HC251D require pull-up or pull-down resistors on unused inputs?

Yes - all unused inputs of the SN74HC251D must be tied to VCC or GND to prevent floating states that cause excessive ICC, oscillation, or undefined outputs. TI's datasheet (Section 5.2 note) mandates this, and application report SCBA004 details risks of slow or floating CMOS inputs. Leaving A, B, C, OE, or any Dn unconnected violates recommended operating conditions and may compromise SN74HC251D reliability.