Texas Instruments CD74HCT221E

Overview

CD74HCT221E from Texas Instruments is a dual monostable multivibrator with independent reset, Schmitt-trigger B inputs, and buffered Q/Q̅ outputs. It operates from 4.5V to 5.5V, supports -55°C to +125°C ambient range, delivers pulse widths from ~140ns to >800µs via external RX/CX networks, and is used in precision timing control for industrial sensor interfaces and legacy TTL-compatible logic systems.

For engineers reviewing the CD74HCT221E datasheet, CD74HCT221E pinout, CD74HCT221E application, or CD74HCT221E equivalent, key selection criteria include its HCT-level LSTTL input compatibility (VIH ≥ 2.0V, VIL ≤ 0.8V), dual independent monostable architecture with edge-selectable triggering (A = trailing, B = leading), and guaranteed operation across extended temperature extremes without derating.

Technical Context

The CD74HCT221E implements two independent monostable circuits, each with separate A (trailing-edge), B (leading-edge), and R (active-low reset) inputs. Timing is externally set by resistor RX and capacitor CX, with pulse width t_W ≈ 0.7·R_XC_X at V_CC = 4.5V. The Schmitt-trigger on B inputs enables robust noise immunity for slow-rising signals.

Each monostable features buffered complementary Q and Q̅ outputs capable of driving up to 15 LSTTL loads over temperature. Propagation delays are tightly matched: t_PLH/t_PHL ≤ 63ns (max) at V_CC = 4.5V, CL = 50pF, and output transition times ≤ 22ns (max), ensuring precise timing alignment between channels.

Key Specifications

Pinout & Package

CD74HCT221E is supplied in a 16-lead plastic dual in-line package (PDIP), with 0.3-inch body width and standard through-hole footprint per JEDEC MS-001. Pin 1 is located at the top-left corner adjacent to the notch or dot marking.

Key Features

Applications

Equivalent & Alternatives

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

Compared with CD74HC221E and SN74LS221N, CD74HCT221E uniquely balances LSTTL input compatibility, extended temperature operation, and Schmitt-trigger noise rejection - making it optimal for retrofitting legacy 5V systems requiring reliability under harsh conditions.

Availability

CD74HCT221E is available at Aetrix Electronics and suitable for industrial sensor interfaces, legacy system timing, power sequencing control, and digital instrumentation requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74HCT221E 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 experience in high-reliability timing and interface ICs.

The CD74HCT221E belongs to TI's High-Speed CMOS Logic family, designed specifically for drop-in replacement of legacy TTL timing functions while delivering lower power, improved noise immunity, and extended temperature operation.

FAQ

What is the minimum external timing capacitor value supported by the CD74HCT221E?

The CD74HCT221E supports a minimum external timing capacitor (CX) value of 0pF, enabling nanosecond-scale pulse generation when paired with low-value resistors (e.g., CX = 28pF, RX = 2kΩ yields t_W ≈ 140ns). This capability makes CD74HCT221E suitable for high-speed digital timing applications where minimal delay is critical.

Does the CD74HCT221E require external pull-up or pull-down resistors on unused inputs?

Yes - per the datasheet, any unused A, B, or R inputs on the CD74HCT221E must be terminated either HIGH or LOW to prevent floating states that could cause erratic triggering or increased power consumption. Leaving inputs unconnected violates the device's specified operating conditions and may compromise timing reliability.

Can the CD74HCT221E operate reliably at 4.5V supply voltage across the full -55°C to +125°C range?

Yes - the CD74HCT221E is fully characterized and guaranteed to meet all AC and DC specifications at V_CC = 4.5V across the full -55°C to +125°C temperature range, including propagation delay (≤63ns max), pulse width accuracy, and input threshold compliance (VIH ≥ 2.0V, VIL ≤ 0.8V).

How does the Schmitt-trigger on the B input improve noise immunity in the CD74HCT221E?

The Schmitt-trigger on the B input of the CD74HCT221E provides hysteresis (~0.8V typical at V_CC = 5V), meaning the rising threshold (V_TH+) and falling threshold (V_TH−) differ significantly. This prevents multiple false triggers from slowly rising or noisy signals - a key advantage when interfacing with mechanical switches, slow op-amp outputs, or EMI-prone sensor lines.

Is the CD74HCT221E pin-compatible with the CD74HC221E?

Yes - the CD74HCT221E and CD74HC221E share identical pinouts, package dimensions, and functional block diagrams. However, they differ electrically: CD74HCT221E uses LSTTL-compatible input thresholds, while CD74HC221E uses CMOS thresholds. Swapping them requires verifying input source compatibility to avoid logic-level misinterpretation.