Freescale Semiconductor, Inc.
Ele c tric a l Sp e c ific a tions
13.6 Power Considerations
The average chip junction temperature, TJ, in °C can be obtained from:
TJ = TA + (PD x θJA) (1)
Where:
TA = ambient temperature in °C
θJA = package thermal resistance, junction to ambient in °C/W
PD = PINT + PI/O
PINT = ICC × VCC = chip internal power dissipation
PI/O = power dissipation on input and output pins (user-determined)
For most applications, PI/O < PINT and can be neglected.
Ignoring PI/O, the relationship between PD and TJ is approximately:
K
PD =
(2)
(3)
TJ + 273°C
Solving equations (1) and (2) for K gives:
= PD x (TA + 273°C) + θJA x (PD)2
where K is a constant pertaining to the particular part. K can be
determined from equation (3) by measuring PD (at equilibrium) for a
known TA. Using this value of K, the values of PD and TJ can be obtained
by solving equations (1) and (2) iteratively for any value of TA.
Technical Data
174
MC68HC705C8A — Rev. 3
Electrical Specifications
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