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| MC9S08QD4_07 Datasheet, PDF (176/202 Pages) Freescale Semiconductor, Inc – 8-Bit HCS08 Central Processor Unit (CPU) | |||
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Appendix A Electrical Characteristics
A.3 Thermal Characteristics
This section provides information about operating temperature range, power dissipation, and package
thermal resistance. Power dissipation on I/O pins is usually small compared to the power dissipation in
on-chip logic and voltage regulator circuits and it is user-determined rather than being controlled by the
MCU design. In order to take PI/O into account in power calculations, determine the difference between
actual pin voltage and VSS or VDD and multiply by the pin current for each I/O pin. Except in cases of
unusually high pin current (heavy loads), the difference between pin voltage and VSS or VDD will be very
small.
Table A-2. Thermal Characteristics
Rating
Operating temperature range
(packaged)
Thermal resistance (single-layer board)
8-pin PDIP
8-pin NB SOIC
Thermal resistance (four-layer board)
8-pin PDIP
8-pin NB SOIC
Symbol
TA
θJA
θJA
Value
TL to TH
â40 to 125
113
150
72
87
Unit
°C
°C/W
°C/W
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.
MC9S08QD4 Series MCU Data Sheet, Rev. 3
176
Freescale Semiconductor
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