Texas Instruments SN74LS221DR

Overview

SN74LS221DR from Texas Instruments is a dual TTL monostable multivibrator with independent negative- and positive-transition-triggered inputs per channel, 70 ns maximum output pulse length at V_CC = 5 V, Schmitt-trigger B-input for jitter-free triggering down to 1 V/s, and overriding clear functionality. It serves as a precision timing element in digital logic systems requiring stable, retriggerable one-shot pulses - e.g., debouncing mechanical switches or generating fixed-width strobes in legacy industrial controllers.

For engineers reviewing the SN74LS221DR datasheet, SN74LS221DR pinout, SN74LS221DR application, or SN74LS221DR equivalent, key selection considerations include its 16-pin SOIC-D package, LS-family TTL compatibility (I_OL = 8 mA, I_OH = −400 µA), external R_ext/C_ext-determined pulse width (0.7·R_extC_ext), ±0.5% unit-to-unit pulse width variation, and operating range of 0°C to 70°C.

Technical Context

The SN74LS221DR implements two independent monostable multivibrators sharing no internal coupling; each features dual trigger inputs (A: positive-edge sensitive, B: Schmitt-trigger negative-edge sensitive) and an overriding active-low CLR input that terminates the output pulse regardless of ongoing input transitions. Its internal latching circuitry provides high immunity to V_CC noise (1.5 V typical) and ensures output stability independent of input transition rate.

Pulse width is defined solely by external R_ext and C_ext components via t_w ≈ 0.7·R_extC_ext, with supported ranges of 1.4 kΩ–100 kΩ for R_ext and 0–1000 µF for C_ext. Output rise/fall times are TTL-compatible and pulse-length-independent, while propagation delays (t_PLH/t_PHL) range from 20 ns to 70 ns depending on trigger source and load conditions.

Key Specifications

Pinout & Package

SN74LS221DR is housed in a 16-pin SOIC (D) package with 1.27 mm pitch, 7.5 mm width, and tape-and-reel packaging (2500 units/reel). Pin assignments match SN54/74123 and SN54/74LS123 devices, enabling direct substitution where retriggering is not required.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS123DR and CD74HC221M, the SN74LS221DR uniquely balances LS-TTL compatibility, built-in Schmitt noise immunity, and non-retriggerable precision timing - making it optimal for legacy 5 V systems where signal integrity and deterministic pulse behavior are prioritized over power savings or retriggering flexibility.

Availability

SN74LS221DR is available at Aetrix Electronics and suitable for industrial control panels, legacy instrumentation, and repair of vintage computing hardware requiring stable component supply with full traceability and long-term lifecycle support.

Supply support for SN74LS221DR 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 U.S.-based semiconductor company founded in 1930, specializing in analog and embedded processing technologies with broad portfolio coverage across industrial, automotive, and communications markets.

The SN74LS221DR belongs to TI's legacy 74LS logic family, designed specifically for robust, pin-compatible upgrades of earlier 74121/74123 monostables in commercial-grade digital systems requiring predictable timing and TTL interoperability.

FAQ

What is the maximum recommended external timing resistance for SN74LS221DR?

The maximum recommended external timing resistance (R_ext) for SN74LS221DR is 100 kΩ. This value ensures jitter-free operation across the full 0°C to 70°C temperature range and 4.75 V to 5.25 V supply voltage window. Using R_ext > 100 kΩ may increase susceptibility to noise-induced timing variation, especially at temperature extremes. The SN74LS221DR datasheet specifies this limit based on empirical characterization of pulse width stability and propagation delay consistency.

Can SN74LS221DR generate pulses shorter than 70 ns?

Yes, SN74LS221DR can generate pulses significantly shorter than 70 ns. With R_ext = 2 kΩ and C_ext = 0, the typical output pulse width is 30 ns and minimum is 20 ns - verified under V_CC = 5 V, T_A = 25°C, and R_L = 2 kΩ load conditions. This capability makes SN74LS221DR suitable for generating precise reset or strobe signals in high-speed digital logic where sub-50 ns timing is required.

Does SN74LS221DR support retriggering during an active output pulse?

No, SN74LS221DR does not support retriggering. Once triggered, the output pulse duration is determined solely by the external R_ext/C_ext network and cannot be extended or restarted by subsequent A or B input transitions. This non-retriggerable behavior distinguishes it from the SN74LS123 and ensures deterministic, fixed-width timing - critical in applications like hardware reset generation where overlapping pulses must be avoided.

What is the purpose of the NC pins on SN74LS221DR?

SN74LS221DR has four NC (No Connection) pins: pins 3, 4, 5, and 6 in the 16-pin SOIC-D package. These pins have no internal connection and must remain unconnected in the PCB layout. They exist due to shared die design with other members of the '221 family (e.g., SN54221J with different pinout) and do not affect electrical performance. Leaving them floating is acceptable; tying them to GND or V_CC is unnecessary and not recommended.

How does the Schmitt-trigger input on SN74LS221DR improve noise immunity?

The Schmitt-trigger input on the B terminal of SN74LS221DR provides hysteresis with V_T+ ≈ 1–2 V and V_T− ≈ 0.7–0.9 V, giving a noise margin of ~1.2 V. This allows reliable triggering from slowly rising or noisy signals (as slow as 1 V/s) without false edges - unlike standard TTL inputs. In practice, this eliminates need for external RC filters in switch debounce or sensor interface applications, reducing component count and improving system reliability in electrically harsh environments.