Peripheral operating requirements and behaviors
Table 8. Thermal characteristics (continued)
Rating
Symbol
Value
Unit
Thermal resistance four-layer board
64-pin LQFP
64-pin QFP
48-pin LQFP
44-pin LQFP
32-pin LQFP
θJA
θJA
θJA
θJA
θJA
53
47
57
53
57
°C/W
°C/W
°C/W
°C/W
°C/W
The average chip-junction temperature (TJ) in °C can be obtained from:
TJ = TA + (PD × θJA)
Where:
TA = Ambient temperature, °C
θJA = Package thermal resistance, junction-to-ambient, °C/W
PD = Pint + PI/O
Pint = IDD × VDD, Watts - chip internal power
PI/O = Power dissipation on input and output pins - 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 the equations above for K gives:
K = PD × (TA + 273 °C) + θJA × (PD)2
where K is a constant pertaining to the particular part. K can be determined by measuring
PD (at equilibrium) for an known TA. Using this value of K, the values of PD and TJ can
be obtained by solving the above equations iteratively for any value of TA.
6 Peripheral operating requirements and behaviors
MC9S08PA60 Series Data Sheet, Rev. 1, 10/9/2012.
Freescale Semiconductor, Inc.
17
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