Texas Instruments CD74HC4538NS

Overview

CD74HC4538NS from Texas Instruments is a dual retriggerable/resettable monostable multivibrator IC used for precision timing in digital systems. It delivers independent trigger/reset propagation delays (e.g., 50 ns at VCC = 4.5 V), supports leading- and trailing-edge triggering via Schmitt-trigger inputs A/B, and generates buffered Q/Q outputs with pulse widths adjustable from microseconds to milliseconds using external RX/CX components. It operates across -55°C to +125°C and is commonly deployed in pulse shaping, debounce circuits, and timing control for industrial PLCs.

For engineers reviewing the CD74HC4538NS datasheet, CD74HC4538NS pinout, CD74HC4538NS application, or CD74HC4538NS equivalent, key selection considerations include its retriggerable monostable behavior, independence of timing delays from RX/CX values, wide 2V–6V supply range, Schmitt-trigger input noise immunity, and SOP-16 package compatibility with space-constrained PCB layouts.

Technical Context

The CD74HC4538NS implements two independent monostable multivibrators, each with separate A (leading-edge) and B (trailing-edge) trigger inputs, dedicated R reset inputs, and dual buffered Q/Q outputs. Its internal architecture uses CMOS logic with Schmitt-trigger inputs on A and B pins, enabling robust noise rejection and consistent edge detection regardless of input slew rate.

Timing is externally controlled by resistor RX and capacitor CX connected to pins 2/14 (1RXCX/2RXCX) and 1/15 (1CX/2CX); output pulse width τ ≈ 0.7 × RX × CX, with RX ≥ 5 kΩ and CX ≥ 0 pF. Propagation delays from A/B→Q and R→Q are independent of RX/CX values and remain stable across temperature and voltage - e.g., tPLH/tPHL ≤ 75 ns at VCC = 6 V, CL = 50 pF.

Key Specifications

Pinout & Package

CD74HC4538NS is housed in a 16-pin SOP (Small Outline Package) with 300 mil body width and 0.65 mm lead pitch, compliant with JEDEC MO-153 and RoHS standards. The package features gull-wing leads, moisture sensitivity level (MSL) 1, and is optimized for surface-mount reflow assembly.

Key Features

Applications

Equivalent & Alternatives

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

Compared with CD74HC4538NS, CD74HC4528M removes retrigger functionality while retaining identical packaging and temperature range, whereas SN74LV4538AD trades extended 6 V operation for enhanced speed and 3.3 V optimization - making each alternative suitable only for narrowly defined design constraints.

Availability

CD74HC4538NS is available at Aetrix Electronics and suitable for industrial automation, automotive electronics, and test equipment requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74HC4538NS 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 high-reliability IC design expertise.

The CD74HC4538NS belongs to TI's HC-series high-speed CMOS logic family, engineered for precision timing applications where retriggerable behavior, wide supply tolerance, and robust noise immunity are essential - especially in harsh-environment industrial controls.

FAQ

What is the minimum external timing resistor value supported by the CD74HC4538NS?

The CD74HC4538NS specifies a minimum external timing resistor RX of 5 kΩ, as confirmed in the Absolute Maximum Ratings section. Using lower values risks exceeding the 30 mA maximum current into pins 2 or 14 (1RXCX/2RXCX), potentially causing timing inaccuracy or device stress. This limit applies across the full -55°C to +125°C operating range and all supply voltages from 2 V to 6 V.

Can the CD74HC4538NS operate reliably at 3.3 V supply voltage?

Yes, the CD74HC4538NS is fully specified for 2 V to 6 V operation, including 3.3 V. At VCC = 3.3 V, its propagation delays remain within datasheet limits (e.g., tPLH ≤ 120 ns at -55°C to +125°C), input hysteresis maintains ~30% of 3.3 V (~1 V), and output drive supports 10 LSTTL loads. No derating or external components are required for 3.3 V use.

How does the CD74HC4538NS handle unused inputs to prevent floating conditions?

Per the datasheet Functional Description, unused A inputs must be tied to GND and unused B inputs to VCC to avoid undefined states. Unused reset inputs and entire unused monostable sections must also be terminated - e.g., R tied to VCC (inactive) or GND (active) depending on desired default state. Leaving any input floating may cause erratic triggering or increased ICC.

Is the CD74HC4538NS pin-compatible with the CD74HCT4538NS variant?

No, the CD74HC4538NS and CD74HCT4538NS are not pin-compatible. While both share the same SOP-16 package outline and pin numbering, their input logic thresholds differ: HC types accept CMOS-level inputs (VIH ≥ 3.15 V at 4.5 V), whereas HCT types require TTL-compatible thresholds (VIH ≥ 2.0 V). Interchanging them may result in unreliable triggering if driven by legacy 5 V TTL sources.

What is the typical pulse width deviation due to temperature variation for the CD74HC4538NS?

The CD74HC4538NS exhibits ≤ ±1% pulse width matching between its two monostable sections over -55°C to +125°C, as measured under identical RX/CX conditions. Absolute pulse width drift is dominated by external component tolerances (e.g., ±5% for standard X7R capacitors), not the IC itself - the internal timing constant K factor remains stable at ~0.7 across temperature and supply voltage per Figure 8–10.