HD74AC Datasheet, PDF (48/52 Pages) Hitachi Semiconductor – HD74AC Series Common Information

Design Considerations
1.000
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0.5000V/dlc
V_IN
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Figure 21 I_CC versus Input Voltage for HD74ACT Devices
The Delta I_CC specification is the increase in I_CC. For each input at V_CC-2.1 V, the Delta I_CC 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 V_CC-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
I_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
I_CC test, greater care must be taken so that other currents will not mask the actual I_CC 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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