9 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

9.1 Application Information

In this application, two 3-input NOR gates are used to create an SR latch as shown in Figure 9-1. The additional

gate can be used for another application, or the inputs can be grounded and the channel left unused.

This device is used to drive the tamper indicator LED and provide one bit of data to the system controller.

When the tamper switch outputs HIGH, the output Q becomes HIGH. This output remains HIGH until the system

controller addresses the event and sends a HIGH signal to the R input which returns the Q output back to LOW.

9.2 Typical Application

System

Controller

Tamper

Switch 1

RA

SA

Q

R1

Tamper

Indicato r

R2

SB

Tamper

Switch 2

RB

Figure 9-1. Typical application schematic

9.2.1 Design Requirements

9.2.1.1 Power Considerations

Ensure the desired supply voltage is within the range specified in the Recommended Operating Conditions. The

supply voltage sets the device's electrical characteristics as described in the Electrical Characteristics.

The supply must be capable of sourcing current equal to the total current to be sourced by all outputs of the

CD74HC27 plus the maximum supply current, ICC, listed in the Electrical Characteristics. The logic device can

only source or sink as much current as it is provided at the supply and ground pins, respectively. Be sure not to

exceed the maximum total current through GND or VCC listed in the Absolute Maximum Ratings.

Total power consumption can be calculated using the information provided in CMOS Power Consumption and

Cpd Calculation.

Thermal increase can be calculated using the information provided in Thermal Characteristics of Standard Linear

and Logic (SLL) Packages and Devices.

CAUTION

The maximum junction temperature, TJ(max) listed in the Absolute Maximum Ratings, is an

additional limitation to prevent damage to the device. Do not violate any values listed in the Absolute

Maximum Ratings. These limits are provided to prevent damage to the device.

CD74HC27, CD54HC27

SCHS132D – AUGUST 1997 – REVISED APRIL 2021

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