rated at a particular maximum operating frequency is dependent upon the power-up
mode. Therefore, the MC68040 is rated at a maximum power dissipation for either the
large or small buffers at a particular frequency. This allows for control of some of the
thermal management upon reset. The following equation provides a rough method to
calculate the maximum power consumption for a chosen output buffer mode:
PD= Maximum Power Dissipation for Output Buffer Mode Selected
PDSB = Maximum Power Dissipation for Small Buffer Mode (All Outputs)
PDLB = Maximum Power Dissipation for Large Buffer Mode (All Outputs)
PINSLB = Number of Pins Large Buffer Mode
PINSCLB = Number of Pins Capable of the Large Buffer Mode
Table 11-1 lists the simplified relationship on the maximum power dissipation for eight
possible configurations of output buffer modes.
Table 11-1. Maximum Power Dissipation for
Output Buffer Mode Configurations
Output Configuration
Data Bus
Address Bus and
Transfer Attributes Control Signals Maximum Power Dissipation
Small* Small Small PDSB
Small Small Large PDSB + (P DLB – P DSB) × 13%
Small Large Small PDSB + (P DLB – P DSB) × 52%
Small Large Large PDSB + (P DLB – P DSB) × 65%
Large Small Small PDSB + (P DLB – P DSB) × 35%
Large Small Large PDSB + (P DLB – P DSB) × 48%
Large Large Small PDSB + (P DLB – PDSB) × 87%
Large Large Large PDSB + (P DLB – P DSB) × 100%
*The MC68LC040 and MC68EC040 only utilize this row of information.
To calculate the specific power dissipation of a design, the termination method of each
signal must be considered. For example, a signal output that is not connected would not
dissipate any additional power if it were configured in the large rather than the small buffer
mode. Since the maximum operating junction temperature is specified as 110°C, the
maximum case temperature (TC) in °C can be obtained from the following equation:
TC = TJ – PD × θJC
TC= Maximum Case Temperature
TJ= Maximum Junction Temperature
PD= Maximum Power Dissipation of the Device
θJC = Thermal Resistance between the Junction of the Die and the Case
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