Texas Instruments SN74LS221N

Overview

SN74LS221N from Texas Instruments is a dual monostable multivibrator IC in 16-pin PDIP package, delivering TTL-compatible output pulse widths from 35 ns to 70 ms via external R_ext/C_ext components. It features independent negative- and positive-transition-triggered inputs per channel, overriding clear functionality, and Schmitt-trigger B-inputs for jitter-free triggering at rates as slow as 1 V/s - used in digital timing control, pulse shaping, and debounce circuits.

For engineers reviewing the SN74LS221N datasheet, SN74LS221N pinout, SN74LS221N application, or SN74LS221N equivalent, key selection criteria include its 70 ms maximum pulse width (at R_ext = 100 kΩ, C_ext = 1 µF), ±0.5% unit-to-unit pulse-width variation, TTL-level input thresholds (V_T+ = 1–2 V, V_T− = 0.7–0.9 V), and compatibility with legacy 74LS logic systems operating at 4.75–5.25 V.

Technical Context

The SN74LS221N implements two independent retriggerable monostable multivibrators using TTL bipolar technology. Each channel accepts either A (positive-edge) or B (Schmitt-trigger negative-edge) inputs, with asynchronous overriding clear (CLR) that terminates output pulses regardless of timing component state.

Pulse width is defined by t_w ≈ 0.7 × R_ext × C_ext, supported across 1.4 kΩ–100 kΩ resistance and 0–1000 µF capacitance ranges. Internal latching ensures immunity to V_CC noise (typically 1.5 V) and stable operation over 0°C to 70°C without external compensation.

Key Specifications

Pinout & Package

SN74LS221N uses a 16-pin plastic dual in-line package (PDIP-N) with 0.3-inch width and through-hole mounting. Pin spacing is 0.1 inch; body dimensions are 0.787 × 0.265 inches (20.0 × 6.7 mm).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS123N and MC14528BDG, the SN74LS221N uniquely combines TTL compatibility, Schmitt-trigger noise immunity, and 70 ms maximum pulse width in a drop-in PDIP package - making it optimal for upgrading legacy timing circuits while maintaining system voltage and signal integrity constraints.

Availability

SN74LS221N is available at Aetrix Electronics and suitable for industrial control timing, keyboard debounce circuits, digital test equipment, and legacy system pulse shaping requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS221N 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 solutions for industrial, automotive, and communications markets.

The SN74LS221N belongs to TI's legacy 74LS logic family, designed specifically for robust, temperature-stable monostable timing in commercial-grade 5 V digital systems where reliability and pin compatibility outweigh power efficiency concerns.

FAQ

What is the maximum output pulse width achievable with SN74LS221N?

The SN74LS221N achieves a maximum output pulse width of 70 ms when configured with R_ext = 100 kΩ and C_ext = 1 µF. This value is confirmed in the datasheet's timing requirements table for SN74LS221 under recommended operating conditions. Pulse width follows t_w ≈ 0.7 × R_ext × C_ext, and the device maintains jitter-free operation across this full range. The SN74LS221N's extended R_ext capability (up to 100 kΩ) enables longer pulses than the SN74LS123N (max 28 ms).

Does SN74LS221N support retriggering during an active output pulse?

Yes, the SN74LS221N supports retriggering: a new trigger applied to either A or B input during an active output pulse will restart the timing interval from that point. This behavior is inherent to its monostable architecture and is verified in the functional description and switching waveforms of the SN74LS221N datasheet. Retriggering occurs independently per channel and is unaffected by the state of the other channel or CLR input unless CLR is asserted.

Can SN74LS221N be used as a direct replacement for SN74LS123N in existing PCBs?

Yes, the SN74LS221N is pinout-compatible with the SN74LS123N and shares identical PDIP-16 packaging, allowing direct PCB replacement. However, the SN74LS221N offers enhanced features including Schmitt-trigger B inputs and extended pulse width (70 ms vs. 28 ms). To ensure proper operation, verify that external R_ext/C_ext values are recalculated for the desired pulse width, and confirm that the B-input polarity matches the original SN74LS123N implementation per the datasheet note on capacitor polarity reversal.

What are the input voltage thresholds for the B input of SN74LS221N?

The B input of the SN74LS221N features Schmitt-trigger hysteresis with a positive-going threshold V_T+ of 1–2 V and a negative-going threshold V_T− of 0.7–0.9 V, specified at minimum V_CC. These values are documented in the electrical characteristics table for SN74LS221 under recommended operating conditions. This 1.2 V typical hysteresis provides strong noise immunity for slow-rising signals - a key differentiator from non-Schmitt monostables like the SN74LS123N.

How does the overriding clear (CLR) function behave in SN74LS221N?

The overriding clear (CLR) input in the SN74LS221N asynchronously forces the corresponding Q output low, terminating any active output pulse immediately - regardless of the state of A, B, or timing components. This behavior is confirmed in the function table and timing diagrams of the SN74LS221N datasheet. CLR operates independently per channel (1CLR controls Channel 1, 2CLR controls Channel 2), and assertion of CLR has priority over all trigger inputs, enabling fail-safe timing abort functionality in safety-critical applications.