Texas Instruments SN74LS221DRE4

Overview

SN74LS221DRE4 from Texas Instruments is a dual monostable multivibrator IC designed for precise, jitter-free pulse generation in TTL-compatible systems. It features two independent one-shot circuits, each with negative- and positive-transition-triggered inputs (A and B), overriding clear (CLR), and external R_ext/C_ext-controlled output pulse width up to 70 ms. Used in timing control, debounce, and waveform shaping within industrial logic systems.

For engineers reviewing the SN74LS221DRE4 datasheet, SN74LS221DRE4 pinout, SN74LS221DRE4 application, or SN74LS221DRE4 equivalent, key selection considerations include its 16-pin SOIC-D package, 0°C to 70°C operating range, 70 ms maximum pulse length, Schmitt-trigger B-input for noise immunity, and compatibility with legacy '121/'123 designs via R/C value adjustment.

Technical Context

The SN74LS221DRE4 implements two independent TTL-compatible monostable multivibrators with latched internal states. Each section uses NAND-gate-based triggering logic and internal compensation to achieve pulse-width stability independent of V_CC and temperature-variation typically <±0.5% across devices with identical R_ext/C_ext.

Its Schmitt-trigger B input provides 1.2 V typical hysteresis and supports slow-edge triggering down to 1 V/s, while A input accepts fast transitions ≥1 V/µs. Output pulse width follows t_w ≈ 0.7 × R_ext × C_ext, with R_ext adjustable from 1.4 kΩ to 100 kΩ and C_ext from 0 to 1000 µF for jitter-free operation over six decades of capacitance.

Key Specifications

Pinout & Package

SN74LS221DRE4 is housed in a 16-pin SOIC (D) package with 1.27 mm pitch, 7.5 mm width, and 10.3 mm length. Pin 1 is located at the top-left corner when viewed from the top with the marking side up and notch or dot oriented left.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar monostable multivibrator applications.

Compared with SN74LS123DR and 74HC123DR, SN74LS221DRE4 uniquely delivers non-retriggerable operation with overriding clear and industry-standard TTL noise margins-making it the only option for deterministic, interrupt-capable timing in legacy industrial control hardware.

Availability

SN74LS221DRE4 is available at Aetrix Electronics and suitable for industrial control timing, keyboard debounce, legacy system upgrades, and pulse preconditioning requiring stable component supply across extended production lifecycles.

Supply support for SN74LS221DRE4 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 founded in 1930, specializing in analog, embedded processing, and logic ICs with broad industrial, automotive, and aerospace qualification.

The SN74LS221DRE4 belongs to TI's legacy TTL logic product line, engineered specifically for robust, deterministic timing in mission-critical industrial and defense systems where reliability and long-term availability outweigh raw speed or power efficiency.

FAQ

What is the maximum output pulse width achievable with SN74LS221DRE4?

The SN74LS221DRE4 supports a maximum output pulse width of 70 ms when configured with R_ext = 100 kΩ and C_ext = 1000 µF. This value is confirmed in the datasheet's timing requirements table for the SN74LS221 variant. Pulse width follows t_w ≈ 0.7 × R_ext × C_ext, and jitter-free operation is maintained across this full range under 0°C to 70°C and 4.75–5.25 V conditions.

Does SN74LS221DRE4 support retriggering during an active output pulse?

No, SN74LS221DRE4 is a non-retriggerable monostable multivibrator. Once triggered, the output pulse proceeds to completion independent of further A or B input transitions. Retriggering is only possible after the pulse ends or is terminated by the overriding clear (CLR) input. This behavior is explicitly defined in the function table and timing diagrams of the SN74LS221DRE4 datasheet.

Can SN74LS221DRE4 replace SN74123 in an existing design?

Yes, SN74LS221DRE4 can replace SN74123 in many cases due to identical pinout and compatible timing architecture. However, the SN74LS221DRE4 uses t_w ≈ 0.7 × R_ext × C_ext, whereas SN74123 uses t_w ≈ 0.45 × R_ext × C_ext; R_ext/C_ext values must be adjusted accordingly. Also, SN74LS221DRE4 adds overriding clear functionality not present in SN74123.

What is the purpose of the NC pins on SN74LS221DRE4?

SN74LS221DRE4 has five NC (no internal connection) pins: 8, 9, 10, 11, and 12. These pins are physically present for package compatibility with other members of the '221 family but carry no internal signal or power connection. Per TI application guidance, they may be left unconnected, tied to GND, or tied to V_CC-no electrical impact results, but grounding improves noise margin in high-EMI environments.

Is SN74LS221DRE4 RoHS compliant?

Yes, SN74LS221DRE4 is RoHS compliant. According to TI's packaging addendum, the SOIC-D package version (including DR and DRE4 suffixes) carries NIPDAU lead finish and meets RoHS Directive 2011/65/EU requirements. The "E4" suffix specifically denotes TI's green packaging standard, confirming compliance with lead-free soldering and hazardous substance restrictions.