Texas Instruments SN74LS221D

Overview

SN74LS221D from Texas Instruments is a dual monostable multivibrator IC designed for precise, jitter-free pulse generation in TTL-compatible digital systems. It features two independent one-shot circuits, each with both 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, debouncing, and delay generation within industrial logic systems.

For engineers reviewing the SN74LS221D datasheet, SN74LS221D pinout, SN74LS221D application, or SN74LS221D 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 (1.2 V typical hysteresis), and compatibility with legacy '121/'123 timing designs via identical pinout.

Technical Context

The SN74LS221D implements two independent TTL-compatible monostable multivibrators, each using internal latching circuitry to isolate output pulse duration from input transition timing. Pulse width is defined by t_w ≈ 0.7 × R_ext × C_ext, with jitter-free operation maintained across 10 pF–1000 µF capacitance and 1.4 kΩ–100 kΩ resistance ranges.

Each section includes a Schmitt-trigger B input (1.2 V typical hysteresis) enabling reliable triggering from slow-rising signals down to 1 V/s, and an A input with 1 V/µs minimum slew rate requirement. The overriding clear (CLR) input terminates the output pulse asynchronously, independent of timing components.

Key Specifications

Pinout & Package

SN74LS221D is housed in a 16-pin SOIC (Small Outline Integrated Circuit) package, 3.9 mm wide, with 1.27 mm pitch and gull-wing leads. Pin 1 is located at the top-left corner (Q1 quadrant per tape orientation), marked by a beveled edge or dot on the package.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS123D and SN74LS221N, the SN74LS221D uniquely combines jitter-free long-pulse capability (70 ms), Schmitt-trigger noise immunity, and SOIC surface-mount compatibility-making it the only choice for modernized industrial timing where reliability and manufacturability intersect.

Availability

SN74LS221D is available at Aetrix Electronics and suitable for industrial control timing, keyboard debounce circuit, legacy system timing upgrade, and test equipment pulse generator applications requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS221D 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 with broad industrial and automotive qualification.

The SN74LS221D belongs to TI's legacy 74LS logic family, engineered for high-noise-immunity TTL-compatible timing in industrial automation, instrumentation, and retro-compatibility applications.

FAQ

What is the maximum output pulse width achievable with SN74LS221D?

The SN74LS221D supports a maximum output pulse width of 70 ms when configured with R_ext = 100 kΩ and C_ext = 1 µF. This value is specified under recommended operating conditions (V_CC = 4.75 V to 5.25 V, T_A = 0°C to 70°C) and reflects the upper limit of its jitter-free timing range. Shorter pulses-from 35 ns (with R_ext = 2 kΩ, C_ext = 0)-are also fully supported. The SN74LS221D achieves this via internal compensation that stabilizes t_w ≈ 0.7 × R_ext × C_ext across voltage and temperature.

Does SN74LS221D have Schmitt-trigger inputs, and which pins feature them?

Yes, the SN74LS221D includes Schmitt-trigger circuitry exclusively on the B inputs (Pin 3 for Section 1, Pin 15 for Section 2). This provides 1.2 V typical hysteresis, enabling reliable triggering from slow-rising or noisy signals as slow as 1 V/s. The A inputs (Pins 1 and 16) do not have Schmitt-triggering and require minimum slew rates of 1 V/µs. This asymmetry allows designers to assign noise-prone signals to B inputs and fast-edge clocks to A inputs-maximizing robustness in mixed-signal environments.

Can SN74LS221D replace SN74LS123 in an existing design without changes?

Yes, the SN74LS221D is pinout-compatible with the SN74LS123D and shares identical pin assignments, allowing direct substitution in most cases. However, two adjustments are required: (1) reverse the polarity of the timing capacitor connected to the R_ext/C_ext terminal, and (2) verify that the longer maximum pulse width (70 ms vs. 28 ms for '123) does not affect downstream timing margins. No PCB layout change is needed-the SN74LS221D fits the same SOIC-16 footprint.

What are the absolute maximum ratings for SN74LS221D supply voltage and input voltage?

The absolute maximum supply voltage (V_CC) for SN74LS221D is 7 V, and the absolute maximum input voltage (V_I) is 7 V for LS-series operation. Exceeding these limits-even momentarily-may cause permanent damage. Under recommended operating conditions, V_CC must be maintained between 4.75 V and 5.25 V, and input voltages must stay within 0 V to V_CC + 0.5 V. These ratings are strictly defined in the SDLS213B datasheet and apply regardless of duty cycle or duration.

How does the overriding clear (CLR) function behave in SN74LS221D?

The CLR input (Pins 4 and 12) provides asynchronous, overriding clear functionality in the SN74LS221D: asserting CLR low immediately forces Q low and Q̅ high, terminating any active output pulse regardless of R_ext/C_ext state or ongoing input transitions. The minimum pulse width for CLR is 40 ns, and setup time before triggering is 15 ns. This behavior is electrically identical across both monostable sections and is critical for fail-safe shutdown in safety-critical logic paths.