8 Detailed Description

8.1 Overview

This device contains two independent 4-input AND gates. Each gate performs the Boolean function Y = A ● B

● C ● D in positive logic.

8.2 Functional Block Diagram

xA

xB

xY

xC

xD

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 Section 6.1 must be followed at all times.

The CD74HC21 can drive a load with a total capacitance less than or equal to the maximum load listed in the

Section 6.6 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 Section 6.1.

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 Section 6.5. The worst case resistance is calculated with the

maximum input voltage, given in the Section 6.1, and the maximum input leakage current, given in the Section

6.5, 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 Section

6.3 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.

CD74HC21, CD54HC21

SCHS131D – AUGUST 1997 – REVISED MAY 2021

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