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MC9S08SG32 Datasheet, PDF (292/328 Pages) Freescale Semiconductor, Inc – HCS08 Microcontrollers | |||
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Appendix A Electrical Characteristics
The average chip-junction temperature (TJ) in °C can be obtained from:
where:
TJ = TA + (PD à θJA)
Eqn. A-1
TA = Ambient temperature, °C
θJA = Package thermal resistance, junction-to-ambient, °C/W
PD = Pint + PI/O
Pint = IDD Ã VDD, Watts â chip internal power
PI/O = Power dissipation on input and output pins â user determined
For most applications, PI/O << Pint and can be neglected. An approximate relationship between PD and TJ
(if PI/O is neglected) is:
PD = K ÷ (TJ + 273°C)
Solving Equation A-1 and Equation A-2 for K gives:
Eqn. A-2
K = PD à (TA + 273°C) + θJA à (PD)2
Eqn. A-3
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 Equation A-1 and Equation A-2 iteratively for any value of TA.
MC9S08SG32 Data Sheet, Rev. 7
292
Freescale Semiconductor
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