Texas Instruments SN74LS221DBR

Overview

SN74LS221DBR from Texas Instruments is a dual monostable multivibrator IC with TTL-compatible inputs and outputs, designed for precise pulse generation in digital timing circuits. 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 pulse width up to 70 ms. Used in clock stretching, debounce, delay generation, and waveform shaping in industrial control and instrumentation systems.

For engineers reviewing the SN74LS221DBR datasheet, SN74LS221DBR pinout, SN74LS221DBR application, or SN74LS221DBR equivalent, key selection considerations include its 16-pin SSOP (DB) package, 0°C to 70°C operating range, 70 ms maximum output pulse length, Schmitt-trigger B-input for jitter-free triggering down to 1 V/s, and compatibility with legacy SN74LS123 layouts via R/C value adjustment.

Technical Context

The SN74LS221DBR implements two independent TTL-level monostable multivibrators using internal latching circuitry that isolates output state from further A/B input transitions once triggered. Pulse width is determined solely by external R_ext and C_ext components per the relationship t_w ≈ 0.7 × R_extC_ext, with stability maintained across temperature and supply voltage variations.

Its Schmitt-trigger B-input provides 1.2 V typical hysteresis and supports slow-edge inputs (≥1 V/s), while A-input accepts fast edges (≥1 V/µs). The overriding clear input terminates output pulses asynchronously, with 40 ns minimum pulse duration and 15 ns setup time relative to inactive-state recovery.

Key Specifications

Pinout & Package

SN74LS221DBR is housed in a 16-pin SSOP (Shrink Small Outline Package) with 0.65 mm lead pitch, 5.3 mm width, and 1.2 mm height - optimized for high-density PCB layouts while maintaining hand-solderability.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS123DR and SN74HC123DR, the SN74LS221DBR delivers superior noise immunity via Schmitt-trigger B-input, extended 70 ms timing range, and tighter ±0.5% pulse-width consistency - making it preferred for industrial debounce and precision delay applications where signal integrity is critical.

Availability

SN74LS221DBR is available at Aetrix Electronics and suitable for industrial control panels, test equipment interfaces, and embedded timing modules requiring stable component supply over extended production lifecycles.

Supply support for SN74LS221DBR 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, embedded processing, and logic ICs with over 90 years of innovation in industrial, automotive, and communications markets.

The SN74LS221DBR belongs to TI's legacy 74LS logic family, engineered for robust TTL-compatible timing functions in noise-prone environments - emphasizing reliability, pin compatibility, and wide R/C programmability over ultra-low power or high-speed performance.

FAQ

What is the maximum output pulse width achievable with SN74LS221DBR?

The SN74LS221DBR supports a maximum output pulse width of 70 ms, attained using R_ext = 100 kΩ and C_ext = 1 µF. This value is specified in the datasheet's timing requirements table for SN74LS221 devices and reflects the upper limit of its jitter-free operation across the full 0°C to 70°C temperature range. Shorter pulses down to 35 ns are also supported with R_ext = 2 kΩ and C_ext = 0.

Does SN74LS221DBR support direct replacement of SN74LS123 in existing PCBs?

Yes, the SN74LS221DBR has identical pinout to SN74LS123, enabling direct PCB replacement. However, the capacitor polarity must be reversed due to internal circuit differences, and R_ext/C_ext values may require adjustment to match desired pulse width, as SN74LS221DBR offers longer maximum timing (70 ms vs. 28 ms) and different timing coefficient behavior.

What is the purpose of the Schmitt-trigger input on the B pin of SN74LS221DBR?

The Schmitt-trigger input on the B pin of SN74LS221DBR provides 1.2 V typical hysteresis, allowing jitter-free triggering from slow-rising or noisy signals as slow as 1 V/s. This feature makes SN74LS221DBR ideal for interfacing with mechanical switches, RC-filtered sensors, or low-slew-rate comparators without external signal conditioning.

Can SN74LS221DBR operate with C_ext = 0 pF for minimal pulse width generation?

Yes, SN74LS221DBR supports C_ext = 0 pF operation, delivering a typical output pulse width of 30 ns with R_ext = 2 kΩ. This mode is explicitly documented in the datasheet and used for generating narrow reset or strobe pulses in high-speed digital systems where external capacitance would degrade timing precision.

What are the absolute maximum ratings for SN74LS221DBR supply voltage and input voltage?

The absolute maximum supply voltage (V_CC) for SN74LS221DBR is 7 V, and the absolute maximum input voltage (V_I) is 7 V for LS-series operation. Exceeding these ratings risks permanent device damage. Recommended operating conditions specify V_CC between 4.75 V and 5.25 V, and input voltages must remain within GND–V_CC during normal operation to ensure reliable function.