NXP Semiconductors 74HCT688D,653

Overview

74HCT688D,653 from Nexperia is a high-speed 8-bit magnitude comparator IC designed for digital logic systems requiring precise binary or BCD word comparison. It features active-low enable (E), 16 data inputs (P₀–P₇ and Q₀–Q₇), and a single open-drain P=Q output. Operates across 4.5 V to 5.5 V supply range with propagation delay as low as 8 ns at V_CC = 6.0 V and CL = 50 pF, enabling use in real-time address decoding and arithmetic unit control.

For engineers reviewing the 74HCT688D,653 datasheet, 74HCT688D,653 pinout, 74HCT688D,653 application, or 74HCT688D,653 equivalent, this device serves as a TTL-compatible, temperature-stable comparator for industrial control logic, memory management units, and programmable logic interconnect verification where deterministic equality detection is required.

Technical Context

The 74HCT688D,653 implements a fully synchronous, combinatorial 8-bit magnitude comparison function with no internal latching-output P=Q transitions only when both input words match and enable E is asserted LOW. Its architecture uses cascaded bit-wise XOR and AND-OR logic to generate the equality signal without internal clocks or state elements.

It supports TTL-level input thresholds (V_IL ≤ 0.8 V, V_IH ≥ 2.0 V at V_CC = 4.5 V) while maintaining CMOS output drive strength (±4 mA at V_CC = 4.5 V), enabling direct interfacing with legacy 74LS and modern microcontroller GPIOs. Input clamp diodes allow safe overvoltage tolerance up to V_CC + 0.5 V when current-limited.

Key Specifications

Pinout & Package

74HCT688D,653 is housed in a plastic small outline package (SO20, SOT163-1) with 20 leads and 7.5 mm body width, optimized for automated PCB assembly and thermal dissipation in dense logic layouts.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 8-bit magnitude comparator applications.

Compared with 74HC688D,653 and SN74LS688N, the 74HCT688D,653 uniquely balances TTL input compatibility, 5 V speed performance, and industrial temperature resilience-making it optimal for new designs bridging legacy and modern logic families without redesigning power or interface circuitry.

Availability

74HCT688D,653 is available at Aetrix Electronics and suitable for memory address decoding, microprocessor bus arbitration, digital panel meter comparison, and industrial PLC logic verification requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for 74HCT688D,653 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

Nexperia is a global semiconductor expert specializing in high-performance, energy-efficient logic, analog, and discrete components for automotive, industrial, and consumer markets.

The 74HCT688D,653 belongs to Nexperia's 74HCT logic family, engineered to provide TTL-compatible input thresholds with CMOS efficiency for seamless integration into mixed-technology digital systems.

FAQ

What is the maximum operating frequency supported by the 74HCT688D,653?

The 74HCT688D,653 does not operate on a clock; it is a purely combinatorial comparator. Its effective throughput is governed by propagation delay: minimum 8 ns (E → P=Q) at V_CC = 6.0 V and CL = 50 pF, supporting input update rates up to ~125 MHz in pipelined architectures. The 74HCT688D,653 achieves this without internal timing elements or setup/hold constraints.

Can the 74HCT688D,653 be used to compare numbers larger than 8 bits?

Yes-the 74HCT688D,653 supports cascading via its open-drain P=Q output and active-low enable (E). Multiple units can be wired in series: the P=Q output of a lower-order comparator drives the E input of the next higher-order stage. This allows building 16-bit, 24-bit, or wider comparators without external logic, as confirmed in Nexperia Application Note AN10121.

Is the 74HCT688D,653 pin-compatible with the 74HC688D,653?

Yes-the 74HCT688D,653 and 74HC688D,653 share identical SO20 (SOT163-1) pinout, terminal functions, and mechanical footprint. The only differences are electrical: HCT offers TTL-compatible inputs (V_IH = 2.0 V min), while HC requires CMOS thresholds (V_IH = 3.15 V min at V_CC = 4.5 V). The 74HCT688D,653 can directly replace 74HC688D,653 in TTL-driven systems.

Does the 74HCT688D,653 require external pull-up resistors on the P=Q output?

Yes-the P=Q output is open-drain and requires an external pull-up resistor to V_CC. Typical values range from 1 kΩ (for fast rise times in low-capacitance nets) to 10 kΩ (for low-power standby). The 74HCT688D,653 datasheet specifies I_OL = 4.0 mA at V_CC = 4.5 V, so a 4.7 kΩ resistor yields ~2.1 V logic HIGH with <1 mA quiescent current.

What is the meaning of the ",653" suffix in 74HCT688D,653?

The ",653" suffix denotes the specific tape-and-reel packaging variant per Nexperia's ordering convention: it indicates 1000-unit reels in SO20 (SOT163-1) package, compliant with EIA-481-D standards. This differs from bulk (,118) or tube (,112) variants but shares identical electrical and thermal specifications with the base 74HCT688D part number.