8 Detailed Description

8.1 Overview

This device contains three independent 3-input NOR gates. Each gate performs the Boolean function

Y = A + B + C in positive logic.

8.2 Functional Block Diagram

xA

xC

xY

xB

8.3 Feature Description

8.3.1 Balanced CMOS Push-Pull Outputs

A balanced output allows the device to sink and source similar currents. The drive capability of this device

may create fast edges into light loads so routing and load conditions should be considered to prevent ringing.

Additionally, the outputs of this device are capable of driving larger currents than the device can sustain without

being damaged. It is important for the output power of the device to be limited to avoid damage due to

over-current. The electrical and thermal limits defined in the Absolute Maximum Ratings must be followed at all

times.

The CD74HC27 can drive a load with a total capacitance less than or equal to the maximum load listed in the

Switching Characteristics connected to a high-impedance CMOS input while still meeting all of the datasheet

specifications. Larger capacitive loads can be applied, however it is not recommended to exceed the provided

load value. If larger capacitive loads are required, it is recommended to add a series resistor between the output

and the capacitor to limit output current to the values given in the Absolute Maximum Ratings.

8.3.2 Standard CMOS Inputs

Standard CMOS inputs are high impedance and are typically modeled as a resistor from the input to ground in

parallel with the input capacitance given in the Electrical Characteristics. The worst case resistance is calculated

with the maximum input voltage, given in the Absolute Maximum Ratings, and the maximum input leakage

current, given in the Electrical Characteristics, using ohm's law (R = V ÷ I).

Signals applied to the inputs need to have fast edge rates, as defined by the input transition time in the

Recommended Operating Conditions to avoid excessive current consumption and oscillations. If a slow or noisy

input signal is required, a device with a Schmitt-trigger input should be used to condition the input signal prior to

the standard CMOS input.

CD74HC27, CD54HC27

SCHS132D – AUGUST 1997 – REVISED APRIL 2021

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