Texas Instruments SN74LS423DR

Overview

SN74LS423DR from Texas Instruments is a dual retriggerable monostable multivibrator IC used for precise pulse generation and timing control in digital logic systems. It features two independent monostable circuits, 15 ns typical propagation delay, 30 ns minimum output pulse width, TTL-compatible inputs, and operates over 0°C to 70°C ambient temperature - commonly deployed in debounce circuits, event timing, and clock stretching applications.

For engineers reviewing the SN74LS423DR datasheet, SN74LS423DR pinout, SN74LS423DR application, or SN74LS423DR equivalent, key selection considerations include guaranteed pulse width stability across temperature, retriggerability without output interruption, input noise immunity, and compatibility with legacy 5-V TTL logic families.

Technical Context

The SN74LS423DR implements two identical edge-triggered monostable multivibrators, each with separate A and B trigger inputs, an active-low clear (CLR), and complementary Q and Q̅ outputs. Retriggering during an active output pulse resets the timing cycle, enabling continuous pulse extension.

Each monostable uses external RC networks to set output pulse width (t_W ≈ 0.3 × R × C), supports both rising- and falling-edge triggering via dual inputs, and maintains output state integrity under transient noise due to Schmitt-trigger input thresholds on A and B pins.

Key Specifications

Pinout & Package

SOP (NS) package, 16-pin surface-mount, 1.27 mm pitch, 10.4 mm × 5.3 mm body, 2.00 mm max height, RoHS-compliant NiPdAu lead finish, MSL Level-1.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS123DR and CD4098BE, the SN74LS423DR uniquely supports retriggerable operation within standard TTL voltage and timing constraints - making it the only choice among the three for applications requiring uninterrupted pulse extension on repeated triggers.

Availability

SN74LS423DR is available at Aetrix Electronics and suitable for keyboard debounce, serial data timing, pulse width modulation prep, and event synchronization requiring stable component supply and long-term commercial-grade availability.

Supply support for SN74LS423DR 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 solutions with over 90 years of innovation in industrial, automotive, and communications markets.

The SN74LS423DR belongs to TI's legacy 74LS logic family, designed specifically for robust, low-power, high-noise-margin timing and control functions in 5-V TTL systems.

FAQ

What is the maximum recommended external resistor value for timing the SN74LS423DR?

The SN74LS423DR datasheet specifies a maximum external timing resistor of 40 kΩ to ensure reliable monostable operation and avoid timing drift due to leakage current. Using higher values risks inconsistent pulse width accuracy and increased sensitivity to PCB contamination or humidity. For stable performance across temperature, TI recommends pairing resistors ≤40 kΩ with capacitors ≥100 pF. The SN74LS423DR's internal timing circuitry is optimized for this range, and exceeding it may cause failure to trigger or erratic output behavior.

Does the SN74LS423DR support both rising- and falling-edge triggering?

Yes, the SN74LS423DR supports both rising- and falling-edge triggering on its A and B inputs per channel. Each monostable uses Schmitt-trigger inputs with hysteresis, allowing reliable edge detection regardless of input slew rate. Triggering occurs on the first valid edge (low-to-high on A or high-to-low on B), and the device remains insensitive to subsequent edges until the output pulse completes - unless retriggered. This dual-edge capability is intrinsic to the SN74LS423DR design and does not require external configuration.

Can the SN74LS423DR operate at 3.3 V?

No, the SN74LS423DR is specified only for 4.75 V to 5.25 V operation and is not functional at 3.3 V. Its internal TTL circuitry requires minimum V_CC of 4.75 V to maintain proper noise margins, output drive strength, and propagation delay. Attempting 3.3 V operation results in undefined outputs, increased propagation delay, and potential failure to trigger. For 3.3 V systems, consider logic-level translators or modern alternatives like the SN74LVC1G123 - but note these are not drop-in replacements for the SN74LS423DR.

What is the function of the CLR pin on the SN74LS423DR?

The CLR (clear) pin on the SN74LS423DR is an asynchronous active-low input that forces the associated monostable's Q output low and Q̅ high, terminating any active output pulse immediately. It overrides ongoing timing cycles and is independent of trigger inputs or RC network values. Both channels have dedicated CLR pins (pin 3 for channel 1, pin 13 for channel 2), enabling independent reset control. This function is essential for system-level fault recovery or forced timing abort in safety-critical applications using the SN74LS423DR.

Is the SN74LS423DR RoHS compliant?

Yes, the SN74LS423DR is RoHS compliant, with NiPdAu (nickel-palladium-gold) lead finish and full compliance verified per TI's RoHS Statement. The "Yes" RoHS designation applies to all active orderable variants including SN74LS423DR, SN74LS423NSR, and SN74LS423D. This compliance covers restriction of lead, mercury, cadmium, hexavalent chromium, PBB, and PBDE substances at or below EU Directive 2011/65/EU thresholds. The SN74LS423DR meets JEDEC J-STD-609A Class 1 marking requirements for lead-free identification.