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MC68HC11KW1 Datasheet, PDF (206/238 Pages) Motorola, Inc – High-density complementary metal oxide semiconductor HCMOS) microcontroller unit
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A.2
Thermal characteristics and power considerations
The average chip junction temperature, TJ, in degrees Celsius can be obtained from the following
equation:
TJ = TA + (PD • θJA)
[1]
where:
TA = Ambient temperature (°C)
θJA = Package thermal resistance, junction-to-ambient (°C/W)
PD = Total power dissipation = PINT + PI/O (W)
PINT = Internal chip power = IDD • VDD (W)
PI/O = Power dissipation on input and output pins (user determined)
An approximate relationship between PD and TJ (if PI/O is neglected) is:
PD = T----J----+--K---2----7---3--
[2]
Solving equations [1] and [2] for K gives:
K = PD • (TA + 273) + θJA • PD2
[3]
where K is a constant for a particular part. K can be determined by measuring PD (at equilibrium)
for a known TA. Using this value of K, the values of PD and TJ can be obtained for any value of TA,
by solving the above equations. The package thermal characteristics are shown below:
A
Characteristics
Thermal resistance
– 100-pin TQFP package
Symbol
θJA
Value
54°
Unit
°C/W
ELECTRICAL SPECIFICATIONS
MC68HC11KW1
A-2
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