HD74AC datasheet(48/52 Pages) HITACHI

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Design Considerations

1.000

0.000

5.000

0.5000V/dlc

VIN

Figure 21 I

CC versus Input Voltage for HD74ACT Devices

The Delta I

CC specification is the increase in ICC. For each input at VCC-2.1 V, the Delta ICC value should be

added to the quiescent supply current to arrive at the circuit’s worst-case static I

CC value.

Fortunately, there are several factors which tend to reduce the increase in I

CC per input. Most TTL devices

will be able to drive FACT inputs well beyond the TTL output specification due to FACT’s low input

loading in a typical system. For example, FAST logic outputs can drive HD74ACT-type inputs down to

200 mV and up to 3.5 V. Additionally, the typical I

CC increase per input will be less than the specified

limit. As shown in the graph above, the I

CC increase at VCC-2.1 V is less than 200 µA in the typical system.

Experiments have shown that the I

CC of an HD74ACT240 series device typically increases only 200 µA

when all of the inputs are connected to a FAST device instead of ground or V

CC.

It is important when designing with FACT, as with any TTL-compatible CMOS technology, that the Delta

CC specification be considered. Designers should be aware of the spec’s significance and that the data

book specification is a worst-case value; most systems will see values that are much less.

7. Testing Advanced CMOS Devices with I/O Pins

There are more and more CMOS families becoming available which can replace TTL circuits. Although

testing these new CMOS units with programs and fixtures which were developed for bipolar devices will

yield acceptable results most of the time, there are some cases where this approach will cause the test

engineer problems.

Such is the case with parts that have a bidirectional pin, exemplified by the HD74AC245/HD74ACT245

octal transceiver. If the proper testing methods are not followed, these types of parts may not pass those

tests for I

CC and input leakage currents, even when there is no fault with the devices.

CMOS circuits, unlike their bipolar counterparts, have static I

CC specification orders of magnitude less than

standard load currents. Most CMOS I

CC specifications are usually less than 100 µA. When conducting an

CC test, greater care must be taken so that other currents will not mask the actual ICC so that other currents

will not mask the actual I

CC of the device. These currents are usually sourced from the inputs and outputs.

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