HD74AC Datasheet, PDF (13/52 Pages) Hitachi Semiconductor – HD74AC Series Common Information

FACT Descriptions and Family Characteristics
On FACT device data sheets, C_PD is a typical value and is given either for the package or for the individual
device (i.e., gates, flip-flops, etc.) within the package.
The circuit shown in Figure 9 was used to compare the power consumption of FACT versus ALS devices.
Two identical circuits were built on the same board and driven from the same input. In the circuit, the
input signal was driven into four D-type flip-flops which act as divide-by-2 frequency dividers. The
outputs from the flip-flops were connected to the inputs of a HD74AC138/74ALS138 decoder. This
generated eight non-overlapping clock pulses on the outputs of the HD74AC138/74ALS138, which were
then connected to an HD74AC04/74ALS04 inverter. The input frequency was then varied and the power
consumption was measured. Figure 10 illustrates the results of these measurements.
400
350
300
250
200
150
100
50
0
0
FACT
ALS
ALS Circuit
Stopped
Functioning
20
40
60
80
100
Frequency (MHz)
Figure 10 FACT vs ALS Circuit Power
Below 40 MHz, the FACT circuit dissipates much less power than the ALS version. It is interesting to note
that when the frequency went to zero, the FACT circuit’s power consumption also went to zero; the ALS
circuit continued to dissipate almost 100 mW. Another advantage of FACT is its capabilities above 40
MHz. At this frequency, the first 74ALS74 D-type flip-flop ceased to operate. Once this occurred, the
entire circuit stopped working and the power consumption fell to its quiescent value. The FACT device,
however, continued functioning beyond the limit of the frequency generator, which was 100 MHz.
This graph shows two advantages of FACT circuits (power and speed). FACT logic delivers increased
performance in addition to offering the power savings of CMOS.
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