One of Two Channels

xA

xB

xY

xC

xD

7

SN74HCS21

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SCLS793A – MARCH 2020 – REVISED JUNE 2020

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8 Detailed Description

8.1 Overview

This device contains two independent 4-input AND gates with Schmitt-trigger inputs. Each gate performs the

Boolean function Y = A

● B ● C ● D in positive logic.

8.2 Functional Block Diagram

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.

8.3.2 CMOS Schmitt-Trigger Inputs

Standard CMOS inputs are high impedance and are typically modeled as a resistor 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).

The Schmitt-trigger input architecture provides hysteresis as defined by ΔVT in the Electrical Characteristics,

which makes this device extremely tolerant to slow or noisy inputs. While the inputs can be driven much slower

than standard CMOS inputs, it is still recommended to properly terminate unused inputs. Driving the inputs slowly

will also increase dynamic current consumption of the device. For additional information regarding Schmitt-trigger

inputs, please see Understanding Schmitt Triggers.

8.3.3 Clamp Diode Structure

The inputs and outputs to this device have both positive and negative clamping diodes as depicted in Figure 8.

CAUTION

Voltages beyond the values specified in the Absolute Maximum Ratings table can

cause damage to the device. The input negative-voltage and output voltage ratings

may be exceeded if the input and output clamp-current ratings are observed.