Datasheet

7–143

Power Consumption

Several factors influence the power consumption of a device: frequency of operation, number of outputs switching, load

capacitance, number of TTL-level inputs, junction temperature, ambient temperature, and thermal resistance of the device. The

maximum operating frequency is limited by the thermal characteristics of the package. TI provides package power-dissipation

information in data sheets under “absolute maximum ratings”. These numbers are calculated using a junction temperature of

150°C and a board trace length of 750 mils (no airflow). Refer to the Package Thermal Considerations application report in

the ABT data book for the relationship between junction temperature and reliability. Traces, power planes, connectors, and

cooling fans play an important role in improving the heat dissipation. Figures 13 through 15 show the typical power

consumption with single- or all-outputs switching. Figure 16 also shows the maximum frequency at which a family can operate

and still meet the V

and V

specifications. No frequency beyond the maximum number is acceptable. Note that all

registered devices were tested based on the clock frequency, and the nonregistered devices were tested based on the

input frequency.