SN74AS27D NOR Gate IC Equivalent & Substitute Parts

Part Overview

The SN74AS27D is a 3-channel NOR gate logic IC manufactured by Texas Instruments in 14-SOIC surface mount packaging. This component implements three independent 3-input NOR logic functions and operates within a 4.5V to 5.5V supply voltage range. The SN74AS27D is classified as obsolete, making substitute parts necessary for new designs and ongoing production requirements. Alternative parts must maintain functional equivalence while meeting current availability and compliance standards.

Key Parameters

Parameter	SN74AS27D
Logic Type	NOR Gate
Number of Circuits	3
Number of Inputs	3
Voltage - Supply	4.5V ~ 5.5V
Current - Output High, Low	2mA, 20mA
Max Propagation Delay @ V, Max CL	5.5ns @ 5V, 50pF
Operating Temperature	0°C ~ 70°C
Package / Case	14-SOIC (0.154", 3.90mm Width)
Product Status	Obsolete
RoHS Status	ROHS3 Compliant

Substitute Part Grouping Explanation

Substitution of the SN74AS27D is determined by the following critical parameters:

Logic Function: Must implement 3-channel, 3-input NOR gate configuration
Circuit Count & Input Configuration: Exactly 3 circuits with 3 inputs per circuit
Supply Voltage Compatibility: Substitute must operate within or encompass the 4.5V to 5.5V range
Output Drive Capability: Current sourcing and sinking must support system requirements
Propagation Delay: Maximum delay specifications must not exceed application timing constraints
Package Compatibility: Surface mount packaging with compatible pinout and footprint
Compliance & Availability: Active product status with current manufacturing and RoHS3 compliance

The SN74HCS27PWR qualifies as a functional substitute based on these parameters. It maintains the identical logic function, circuit count, and input configuration while offering extended supply voltage range (2V ~ 6V), improved quiescent current characteristics, and active product status with current availability.

Parameter Comparison

Parameter	SN74AS27D	SN74HCS27PWR
Logic Type	NOR Gate	NOR Gate
Number of Circuits	3	3
Number of Inputs	3	3
Voltage - Supply	4.5V ~ 5.5V	2V ~ 6V
Current - Output High, Low	2mA, 20mA	7.8mA, 7.8mA
Max Propagation Delay @ V, Max CL	5.5ns @ 5V, 50pF	12ns @ 6V, 50pF
Operating Temperature	0°C ~ 70°C	-40°C ~ 125°C
Package / Case	14-SOIC (0.154", 3.90mm Width)	14-TSSOP (0.173", 4.40mm Width)
Product Status	Obsolete	Active
RoHS Status	ROHS3 Compliant	ROHS3 Compliant

Engineering Selection Recommendations

The SN74HCS27PWR is the designated manufacturer substitute for the obsolete SN74AS27D. Selection of this substitute is based on:

Functional Equivalence: Identical NOR gate logic implementation with matching circuit and input configuration
Supply Voltage: The extended 2V to 6V range encompasses the original 4.5V to 5.5V specification, ensuring compatibility in standard 5V logic systems
Active Product Status: The SN74HCS27PWR maintains current manufacturing status and full availability, eliminating obsolescence risk
Compliance Certification: Both parts maintain ROHS3 compliance and REACH unaffected status, meeting regulatory requirements
Output Drive Capability: The SN74HCS27PWR provides increased output current (7.8mA vs. 2mA/20mA), offering improved drive capability for downstream logic loads
Temperature Range: Extended operating range (-40°C to 125°C) provides broader environmental coverage compared to the original specification

Package transition from 14-SOIC to 14-TSSOP requires PCB layout modification. Both packages maintain 14-pin configuration with compatible pinout for NOR gate logic functions.

Frequently Asked Questions (FAQ)

Q: Can the SN74HCS27PWR directly replace the SN74AS27D without circuit modification?

A: The SN74HCS27PWR maintains identical logic function and pinout compatibility. However, the package transition from 14-SOIC to 14-TSSOP requires PCB footprint redesign. No circuit topology changes are necessary if the new package footprint is accommodated in the layout.

Q: What are the key differences in electrical performance between these parts?

A: The SN74HCS27PWR offers extended supply voltage range (2V to 6V vs. 4.5V to 5.5V), higher output drive current (7.8mA vs. 2mA/20mA), and wider operating temperature range (-40°C to 125°C vs. 0°C to 70°C). Propagation delay increases from 5.5ns to 12ns, which may impact high-speed timing-critical applications.

Q: Is the SN74HCS27PWR suitable for 5V logic systems?

A: Yes. The 2V to 6V supply voltage range fully encompasses standard 5V logic operation. The part is qualified for use in 5V systems with no additional design considerations beyond package footprint accommodation.

Q: What is the significance of the Schmitt Trigger Input feature in the SN74HCS27PWR?

A: Schmitt Trigger inputs provide hysteresis on the input thresholds, improving noise immunity and signal integrity in systems with slow-rising or noisy input signals. This feature enhances reliability compared to standard input thresholds.

Q: Are there any compliance or certification differences between the two parts?

A: Both parts maintain ROHS3 compliance and REACH unaffected status. No compliance differences exist. The SN74HCS27PWR benefits from active product status, ensuring continued availability and manufacturing support.

Q: What packaging considerations apply when transitioning from SN74AS27D to SN74HCS27PWR?

A: The transition requires PCB redesign to accommodate the 14-TSSOP package (0.173" width) in place of the 14-SOIC package (0.154" width). Pin configuration and logic function remain identical. Tape & Reel packaging for the substitute part supports automated assembly processes.

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