Texas Instruments CD74HCT221M

Overview

CD74HCT221M 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, delivers pulse widths from ~140 ns to >800 µs via external RX/CX networks, and is used in precision timing control for industrial sensor interfaces and power supply sequencing.

For engineers reviewing the CD74HCT221M datasheet, CD74HCT221M pinout, CD74HCT221M application, or CD74HCT221M equivalent, key selection criteria include HCT-level LSTTL input compatibility (VIH ≥ 2.0 V, VIL ≤ 0.8 V), dual independent mono functionality with edge-selectable triggering (A = rising-edge, B = falling-edge), and SOIC-16 package thermal performance (θJA = 73°C/W).

Technical Context

The CD74HCT221M implements two independent retriggerable monostable circuits, each with separate A (rising-edge) and B (falling-edge) trigger inputs, dedicated R (active-low reset), and complementary Q/Q̅ outputs. Timing is externally set by resistor RX and capacitor CX, with pulse width t_W ≈ 0.7·R_XC_X at V_CC = 4.5 V.

Each monostable features Schmitt-triggered B inputs for noise immunity, overriding reset capability that terminates output pulses early, and fanout of 10 LSTTL loads. Propagation delays are 34–51 ns (t_PHL/t_PLH, CL = 50 pF, V_CC = 4.5 V), with matched pulse width tolerance ±2% between internal channels.

Key Specifications

Pinout & Package

CD74HCT221M is housed in a 16-pin SOIC (D) package (7.4 mm × 10.0 mm, 1.2 mm max height), with JEDEC MO-153 compliance and moisture sensitivity level 1 (260°C peak reflow).

Key Features

Applications

Equivalent & Alternatives

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

Compared with CD74HC221M and SN74LS221N, CD74HCT221M uniquely combines LSTTL input compatibility, Schmitt-triggered B inputs, ±2% inter-channel pulse match, and overriding reset - making it optimal for robust, mixed-signal timing in harsh environments.

Availability

CD74HCT221M is available at Aetrix Electronics and suitable for industrial sensor interface, power supply sequencing, motor control fault recovery, and test equipment pulse generation requiring stable component supply across extended temperature ranges.

Supply support for CD74HCT221M 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 delivering analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

CD74HCT221M belongs to TI's High-Speed CMOS Logic family, designed specifically for replacing legacy TTL timing functions with lower power, wider temperature range, and enhanced noise immunity while maintaining pin-for-pin compatibility.

FAQ

What is the minimum external resistor value supported by CD74HCT221M?

The CD74HCT221M specifies a minimum external timing resistor (RX) of 500 Ω. Using values below this may cause timing inaccuracies or failure to trigger reliably due to internal circuit loading effects. This value is confirmed in the Functional Description section of the SCHS166F datasheet and applies across the full -55°C to +125°C operating range for CD74HCT221M.

Does CD74HCT221M support retriggering during an active output pulse?

Yes, CD74HCT221M supports retriggering: a new trigger on either A or B input during an active Q output pulse will restart the timing cycle from zero, extending the total output duration. This behavior is explicitly defined in the Functional Description and Truth Table of the CD74HCT221M datasheet (SCHS166F), and applies independently to each of the two monostable sections.

Can CD74HCT221M operate at 3.3 V supply voltage?

No, CD74HCT221M is specified only for 4.5 V to 5.5 V operation. At 3.3 V, input thresholds (VIH ≥ 2.0 V, VIL ≤ 0.8 V) and output drive levels fall outside guaranteed specifications, and the device may fail to recognize TTL inputs or deliver valid logic levels. For 3.3 V systems, consider CD74HC221M instead - but note its VIH/VIL scale with VCC.

What is the purpose of the Schmitt trigger on the B inputs of CD74HCT221M?

The Schmitt trigger on CD74HCT221M's B inputs provides hysteresis (typically 0.8 V threshold difference), enabling reliable triggering from slow-rising, noisy, or degraded waveforms - such as those from mechanical switches, long PCB traces, or unbuffered sensors. This eliminates false triggering and eliminates need for external RC filtering, a feature absent on the A inputs and non-Schmitt variants like SN74LS221N.

How does the pulse width matching specification apply to CD74HCT221M?

CD74HCT221M guarantees ±2% pulse width match between its two internal monostable circuits when configured identically (same RX, CX, VCC, temperature). This is measured under conditions specified in the Switching Specifications table (CX = 1000 pF, RX = 10 kΩ, VCC = 4.5–5.5 V) and enables precise differential timing, redundancy validation, or phase-aligned dual outputs - a capability not documented for SN74LS221N or basic HC variants.