Texas Instruments SN74LS221NG4

Overview

SN74LS221NG4 from Texas Instruments is a dual TTL monostable multivibrator with independent negative- and positive-transition-triggered inputs per section, 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 synchronization, reset generation, and pulse shaping circuits.

For engineers reviewing the SN74LS221NG4 datasheet, SN74LS221NG4 pinout, SN74LS221NG4 application, or SN74LS221NG4 equivalent, key selection criteria include external R/C programmability (R_ext = 1.4–100 kΩ, C_ext = 0–1000 µF), ±0.5% unit-to-unit pulse-width variation, TTL-compatible outputs, and operation across 0°C to 70°C.

Technical Context

The SN74LS221NG4 implements two independent monostable multivibrators sharing no internal coupling. Each section uses NAND-gate-based latching with separate A (positive-edge) and B (Schmitt-trigger negative-edge) inputs, enabling flexible trigger polarity selection and inhibit capability. The overriding clear input forces Q low regardless of timing state.

Pulse width is defined by t_w ≈ 0.7 × R_ext × C_ext, with stability maintained over full temperature and supply ranges due to internal compensation. Timing independence from V_CC and temperature is confirmed by <±0.5% variation across 4.75–5.25 V and 0–70°C.

Key Specifications

Pinout & Package

SN74LS221NG4 is supplied in a 16-pin plastic dual in-line package (PDIP-N) with 0.3-inch width and through-hole mounting. Pin 1 is located at the top-left corner when the notch faces upward.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS123N and SN74HC123N, the SN74LS221NG4 uniquely combines Schmitt-trigger noise immunity, TTL-compatible thresholds, and guaranteed 70 ns maximum pulse width-making it the only option for robust timing in electrically noisy industrial environments with legacy 5 V logic.

Availability

SN74LS221NG4 is available at Aetrix Electronics and suitable for industrial control sequencing, digital logic synchronization, and power-on reset generation requiring stable component supply across extended production lifecycles.

Supply support for SN74LS221NG4 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 specializing in analog and embedded processing technologies with over 90 years of innovation in industrial, automotive, and communications markets.

The SN74LS221NG4 belongs to TI's legacy 74LS logic family, engineered specifically for high-noise-margin, temperature-stable timing in industrial and military-grade digital systems where reliability under electrical stress is critical.

FAQ

What is the maximum output pulse length achievable with SN74LS221NG4?

The SN74LS221NG4 delivers a maximum output pulse length of 70 ns when R_ext = 2 kΩ and C_ext = 0, but supports extended durations up to milliseconds using external timing components. With R_ext = 10 kΩ and C_ext = 1 µF, the SN74LS221NG4 achieves approximately 7 ms output pulses while maintaining jitter-free operation across its full operating temperature range.

Does SN74LS221NG4 support retriggering during an active output pulse?

No, the SN74LS221NG4 does not support retriggering during an active output pulse. Once triggered, the output remains stable and independent of further A or B input transitions until the pulse completes or is terminated by the overriding clear input. This non-retriggerable behavior ensures deterministic timing intervals critical for reset and gating applications.

Can SN74LS221NG4 be used with capacitors larger than 10 µF?

Yes, the SN74LS221NG4 supports external timing capacitors up to 1000 µF, as confirmed in the datasheet's timing requirements table. Using C_ext = 1000 µF with R_ext = 1.4 kΩ yields pulse widths exceeding 1 second, though accuracy degrades beyond 100 µF due to capacitor leakage and ESR effects.

What is the input hysteresis value of the B input on SN74LS221NG4?

The B input of the SN74LS221NG4 features Schmitt-trigger circuitry with typical hysteresis of 1.2 V, calculated as V_T+ − V_T− = 2 V − 0.8 V. This hysteresis enables jitter-free triggering from slowly rising inputs (as slow as 1 V/s) and provides strong immunity to noise superimposed on the input signal.

Is SN74LS221NG4 pin-compatible with SN74LS123N?

Yes, the SN74LS221NG4 is pin-compatible with SN74LS123N, sharing identical pin assignments and package footprint. However, functional differences exist: the SN74LS221NG4 includes Schmitt-trigger B-inputs and higher timing accuracy, whereas SN74LS123N lacks hysteresis and exhibits greater pulse-width variation across temperature and supply voltage.