HD74AC datasheet(19/52 Pages) HITACHI

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Definition of Specifications

1. Power Dissipation-Test Philosophy

In an erfort to reduce confusion about measuring C

PD, a JEDEC standard test procedure (7A Appendix E)

has been adopted, which specifies the test setup for each type of device. This allows a device to be

exercised in a consistent manner for the purpose of specification comparison. All device measurements are

made with V

CC = 5.0 V at 25°C, with 3-state outputs both enabled and disabled.

• Gates: Switch one input. Bias the remaining inputs such that the output switches.

• Latches: Switch the Enable and D inputs such that the latch toggles.

• Flip-Flops: Switch the clock pin while changing D (or bias J and K) such that the output (s) change each

clock cycle. For parts with a common clock, exercise only one flip - flop.

• Decoders: Switch one address pin which changes two outputs.

• Multiplexers: Switch one address pin with the corresponding data inputs at opposite logic levels so that

the output switches.

• Counters: Switch the clock pin with other inputs biased such that the device counts.

• Shift Registers: Switch the clock pin with other inputs biased such that the device counts.

• Transceivers: Switch one data input. For bidirectional devices enable only one direction.

• Parity Generator: Switch one input.

• Priority Encoders: Switch the lowest priority input.

• Load Capacitance: Each output which is switching should be loaded with the standard 50 pF. The

equivalent load capacitance, based upon the number of outputs switching and their

respective frequency, is then subtracted form the measured gross C

PD number to

obtain the device’s actual C

PD value.

If the device is tested at a high enough frequency, the static supply current can be ignored. Thus at 1 MHz,

the following formula can be used to calculate C

PD:

PD = ICC/ (VCC) (1 × 10

6) – Equivalent Load Capacitance

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