Texas Instruments SN74LS123DB

Overview

SN74LS123DB from Texas Instruments is a dual retriggerable monostable multivibrator IC in SSOP-16 package, operating from 4.75 V to 5.25 V, with typical output pulse width adjustable from 10 ns to 10 s via external RC networks, and TTL-compatible inputs/outputs. It serves as a precision timing element in digital logic systems requiring glitch suppression, debouncing, or event stretching - for example, in keyboard scan controllers or interrupt signal conditioning.

For engineers reviewing the SN74LS123DB datasheet, SN74LS123DB pinout, SN74LS123DB application, or SN74LS123DB equivalent, key selection considerations include its retriggerable behavior (vs. non-retriggerable variants), guaranteed propagation delay of ≤45 ns at V_CC = 5 V, temperature range of 0°C to 70°C, and SSOP packaging for high-density PCB layouts.

Technical Context

The SN74LS123DB implements two independent retriggerable monostable multivibrators, each with separate A and B trigger inputs (active-low and active-high), reset input (active-low), and Q/Q̅ complementary outputs. Its internal circuitry uses bipolar TTL logic with Schmitt-trigger input thresholds on the B inputs to reject noise.

Each monostable's output pulse width is determined solely by an external resistor (R) and capacitor (C), following t_W ≈ 0.33 × R × C, with guaranteed minimum pulse width of 10 ns and no upper limit imposed by internal design - enabling flexible timing from nanoseconds to seconds.

Key Specifications

Pinout & Package

SN74LS123DB is housed in a 16-pin Shrink Small Outline Package (SSOP-DB), 5.3 mm × 6.2 mm body size, 0.65 mm lead pitch, 2.00 mm max height, RoHS-compliant with NiPdAu lead finish, and MSL Level-1 rating.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS123DR and SN74LS123N, the SN74LS123DB offers the smallest PCB footprint and highest assembly density, making it optimal for space-constrained embedded controllers - though it demands fine-pitch reflow capability and tighter solder mask tolerances.

Availability

SN74LS123DB is available at Aetrix Electronics and suitable for industrial control panels, test instrumentation, legacy system upgrades, and embedded interface modules requiring stable component supply across extended production lifecycles.

Supply support for SN74LS123DB 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, automotive, and consumer reach.

The SN74LS123DB belongs to TI's legacy 74LS logic family, designed specifically for robust, low-power TTL-compatible timing functions in cost-sensitive, reliability-critical digital systems.

FAQ

What is the maximum recommended RC time constant for stable operation of SN74LS123DB?

The SN74LS123DB supports RC time constants up to 10 s without degradation in pulse width accuracy or retriggering fidelity. For values above 100 ms, use low-leakage capacitors (e.g., polypropylene or C0G ceramics) and shielded routing to prevent stray capacitance and leakage current effects that could distort timing. The SN74LS123DB maintains specified performance across this full range when layout best practices are followed.

Does SN74LS123DB support asynchronous reset during an active output pulse?

Yes - the SN74LS123DB features an active-low asynchronous reset input (pin 3 for Channel 1, pin 13 for Channel 2) that forces Q low and Q̅ high immediately, regardless of ongoing timing cycle or retrigger events. This allows deterministic termination of output pulses for safety-critical sequencing or emergency stop logic in real-time control systems using the SN74LS123DB.

Can SN74LS123DB operate reliably at 4.5 V supply voltage?

No - the SN74LS123DB is specified only for 4.75 V to 5.25 V operation per its datasheet. At 4.5 V, propagation delays increase significantly, output drive weakens below TTL-compatible levels, and retriggering margin degrades. For 4.5 V systems, consider SN74LV123A or SN74LVC1G123 instead - the SN74LS123DB must be used within its rated V_CC window to guarantee performance.

How does the Schmitt-trigger input on SN74LS123DB's B input improve noise immunity?

The B input (pins 2 and 14) of the SN74LS123DB incorporates built-in hysteresis (~0.5 V), meaning the rising threshold is ~1.7 V and falling threshold is ~1.2 V. This prevents multiple triggering from slowly rising or noisy edges - critical in industrial environments where SN74LS123DB interfaces with relay contacts or long cable runs susceptible to EMI.

Is SN74LS123DB pin-compatible with SN74LS122?

No - SN74LS123DB and SN74LS122 are not pin-compatible. The SN74LS122 is a non-retriggerable dual monostable in 14-pin packages (SOIC-D, PDIP-N), lacking dedicated reset inputs and having different pin assignments. Substituting SN74LS122 for SN74LS123DB requires PCB redesign and logic revalidation - the SN74LS123DB's 16-pin SSOP layout and retriggerable architecture are distinct by design.