Texas Instruments CD74HCT85E

Overview

CD74HCT85E from Texas Instruments is a 4-bit high-speed CMOS magnitude comparator IC designed for digital logic systems requiring precise binary, BCD, or monotonic code comparison. It delivers A > B, A < B, and A = B outputs with 15 ns typical propagation delay (VCC = 5 V, CL = 15 pF), operates across –55°C to +125°C, and supports serial/parallel cascading without external gating in industrial control and test equipment.

For engineers reviewing the CD74HCT85E datasheet, CD74HCT85E pinout, CD74HCT85E application, or CD74HCT85E equivalent, key selection criteria include LSTTL-compatible input thresholds (VIH ≥ 2 V, VIL ≤ 0.8 V), 4.5–5.5 V supply range, PDIP-16 package compatibility, and guaranteed operation under extended temperature extremes.

Technical Context

The CD74HCT85E implements a fully buffered 4-bit comparator architecture with weighted inputs (A0–A3, B0–B3) and three active-high magnitude result outputs. Its internal logic supports both single-device operation and hierarchical expansion via cascading inputs (A>B IN, A<B IN, A=B IN) for multi-word comparisons up to 12 bits or more using serial or parallel topologies.

It uses silicon-gate CMOS technology to achieve LSTTL-equivalent speed while maintaining CMOS-level static power consumption (ICC ≤ 160 μA at TA = –55°C to +125°C). Input compatibility is strictly defined for LSTTL logic levels, with no HC-type 2–6 V operation - only 4.5–5.5 V nominal supply is valid per recommended conditions.

Key Specifications

Pinout & Package

CD74HCT85E is housed in a 16-pin plastic dual in-line package (PDIP-N) with 19.31 mm × 6.35 mm body size and 2.54 mm lead pitch. The package is through-hole mountable, RoHS-compliant (NIPDAU finish), and rated for manual and wave soldering (no MSL restriction).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 4-bit magnitude comparator applications.

Compared with SN74LS85N, CD74HCT85E reduces supply current by >99% and extends operating temperature by 125°C; compared with CD74HC85E, it trades supply flexibility for guaranteed LSTTL input recognition - critical in legacy 5 V logic upgrades.

Availability

CD74HCT85E is available at Aetrix Electronics and suitable for industrial PLC logic modules, digital test equipment, avionics data acquisition, and automotive engine control units requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74HCT85E 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 over 90 years of innovation in high-reliability components.

The CD74HCT85E belongs to TI's legacy HCT logic family, engineered specifically for seamless migration from bipolar TTL systems into lower-power CMOS-based designs while preserving pinout, timing, and interface compatibility.

FAQ

What is the maximum operating frequency supported by CD74HCT85E?

The CD74HCT85E does not specify a maximum clock frequency, as it is a combinational logic device with propagation delay rather than a clocked circuit. Its 15 ns typical propagation delay (An/Bn to A>B OUT at VCC = 5 V, CL = 15 pF) supports reliable operation in systems with signal transition intervals exceeding ~30 ns - effectively enabling use in designs with functional update rates up to approximately 33 MHz. Actual system-level timing depends on board trace capacitance and load conditions.

Does CD74HCT85E require external pull-up or pull-down resistors on its inputs?

No, CD74HCT85E does not require external pull-up or pull-down resistors on its inputs. All inputs (A0–A3, B0–B3, and cascading inputs) have defined logic thresholds and internal protection structures. However, unused inputs must be actively tied to VCC or GND per TI layout guidelines to prevent floating states that could cause increased ICC or erratic output behavior - this is a design requirement, not a resistor dependency.

Can CD74HCT85E be used in a 3.3 V system?

No, CD74HCT85E is not specified for 3.3 V operation. Its recommended operating supply range is strictly 4.5 V to 5.5 V, and its LSTTL-compatible input thresholds (VIH ≥ 2.0 V, VIL ≤ 0.8 V) assume a 5 V reference. Using it at 3.3 V risks failure to recognize high-level inputs reliably and violates the manufacturer's recommended conditions - for 3.3 V systems, consider the CD74LVC85 or similar LVCMOS-compatible comparators instead.

How many CD74HCT85E devices are needed to compare two 12-bit words?

Three CD74HCT85E devices are required to compare two 12-bit words. Each CD74HCT85E handles 4 bits, and they are cascaded using the (A>B) IN, (A<B) IN, and (A=B) IN inputs. The most significant device receives the cascading outputs from the middle device, which in turn receives from the least significant device - enabling full 12-bit magnitude evaluation with no external logic gates, as confirmed in TI's Figure 6-3 (Series Cascading) and Figure 6-4 (Parallel Cascading).

Is CD74HCT85E pin-compatible with CD74HC85E?

Yes, CD74HCT85E is pin-compatible and functionally identical to CD74HC85E in terms of pin assignment, package outline (PDIP-16), and logical behavior. Both share identical pin configuration (A0–A3, B0–B3, cascading inputs/outputs, VCC, GND). However, they differ electrically: CD74HCT85E supports only 4.5–5.5 V operation with LSTTL-compatible inputs, whereas CD74HC85E operates from 2–6 V with CMOS-input thresholds - making them interchangeable only when supply and driving source conditions match the respective type's specifications.