Texas Instruments CD74HC423NSR

Overview

CD74HC423NSR from Texas Instruments is a dual retriggerable monostable multivibrator with independent resets, designed for precise pulse generation and timing control in digital systems. It operates from 2V to 6V, features Schmitt-trigger inputs on both A and B trigger pins, supports wide output-pulse width adjustment via external RX/CX (t_W = 0.45·R_XC_X at V_CC = 5V), and delivers buffered Q and Q̅ outputs. It is used in edge-triggered delay circuits, debounce logic, and programmable one-shot timing in industrial control interfaces.

For engineers reviewing the CD74HC423NSR datasheet, CD74HC423NSR pinout, CD74HC423NSR application, or CD74HC423NSR equivalent, key selection criteria include its retriggerable architecture, separate reset per monostable, trailing/leading-edge triggering capability, -55°C to +125°C operating range, and SOP-16 package compatibility with high-reliability embedded timing designs.

Technical Context

The CD74HC423NSR implements two independent monostable multivibrators, each with dedicated A (trailing-edge), B (leading-edge), and R (active-low reset) inputs. Unlike the '123 family, it lacks negative-to-positive reset pulse triggering - only low-level reset termination is supported. Timing is externally set by resistor R_X (≥5 kΩ min) and capacitor C_X (0 pF min), enabling pulse widths from ~100 ns to >100 ms depending on component values.

Each monostable provides complementary buffered Q and Q̅ outputs with balanced propagation delays (e.g., 64 ns max t_PLH/t_PHL at V_CC = 4.5V, CL = 50 pF) and transition times (≤19 ns). Input hysteresis ensures noise immunity, and fanout supports up to 10 LSTTL loads over full temperature range.

Key Specifications

Pinout & Package

SOP-16 (NS) package: 10.4 mm × 5.3 mm body, 1.27 mm pitch, 2.00 mm max height, gull-wing leads, RoHS-compliant NiPdAu finish, MSL Level-1.

Key Features

Applications

Equivalent & Alternatives

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

Compared with CD74HC123NSR, the CD74HC423NSR trades reset-edge flexibility for simplified reset control; compared with SN74LV123ADR, it sacrifices speed and power efficiency for superior noise immunity and extended temperature operation - making it the preferred choice for ruggedized timing in harsh environments.

Availability

CD74HC423NSR is available at Aetrix Electronics and suitable for industrial automation, aerospace avionics, and energy metering applications requiring stable component supply across extended temperature ranges and long product lifecycles.

Supply support for CD74HC423NSR 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 decades of heritage in high-reliability logic families.

The CD74HC423NSR belongs to TI's HC-series high-speed CMOS logic portfolio, engineered for low-power, wide-voltage, and extended-temperature operation in mission-critical industrial and defense timing applications.

FAQ

What is the minimum external timing capacitor value supported by the CD74HC423NSR?

The CD74HC423NSR supports an external timing capacitor C_X down to 0 pF, meaning it can operate with only an external resistor R_X (≥5 kΩ) for minimal pulse widths. This allows nanosecond-range timing when combined with low-capacitance PCB traces or discrete capacitors. The CD74HC423NSR datasheet confirms C_X = 0 pF as valid in absolute maximum ratings and timing calculations.

Does the CD74HC423NSR support both rising- and falling-edge triggering on the same input pin?

No. The CD74HC423NSR assigns dedicated edge sensitivity per input: pin 1A/2A responds only to trailing (falling) edges, while pin 1B/2B responds only to leading (rising) edges. This separation prevents ambiguity and enables deterministic synchronization to either edge of the same waveform without additional logic - a core architectural feature confirmed in the functional diagram and truth table of the CD74HC423NSR datasheet.

Can unused inputs on the CD74HC423NSR be left floating?

No. Per the CD74HC423NSR datasheet, all unused inputs - including A, B, and R on the inactive monostable - must be terminated either high or low. Floating CMOS inputs risk increased supply current, erratic triggering, or latch-up due to undefined voltage levels. Proper termination ensures predictable behavior, ESD robustness, and compliance with TI's recommended handling procedures for the CD74HC423NSR.

What is the maximum output pulse width achievable with the CD74HC423NSR using practical component values?

Using standard 10 MΩ resistors and 100 µF tantalum capacitors, the CD74HC423NSR achieves pulse widths exceeding 450 seconds (t_W ≈ 0.45 × 10 MΩ × 100 µF). The CD74HC423NSR datasheet specifies no upper limit on R_X or C_X, and leakage current remains negligible within this range. This enables ultra-long timing intervals without external microcontrollers - a key advantage of the CD74HC423NSR in battery-powered or maintenance-free systems.

How does the reset functionality differ between CD74HC423NSR and CD74HC123NSR?

The CD74HC423NSR accepts only active-low reset signals (low level terminates output pulse), whereas the CD74HC123NSR additionally supports active-high reset pulses (positive-going edge terminates pulse). This difference is explicitly stated in the CD74HC423NSR datasheet description: "the 423 types do not have this feature." Designers selecting the CD74HC423NSR must ensure reset sources can drive and hold R pins low for reliable pulse termination.