Freescale Semiconductor, Inc.
GENERAL RELEASE SPECIFICATION
13.4 Thermal Characteristics
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2
Characteristic
Symbol
Value
Unit
Thermal Resistance
Plastic
θJA
60
60
°C/W
SOIC
3
13.5 Power Considerations
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The average chip-junction temperature, TJ, in °C, can be obtained from:
TJ = TA + (PD × θJA)
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(1)
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where:
TA = Ambient temperature, °C
θJA = Package thermal resistance, junction to ambient, °C/W.
PD = PINT + PI/O
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PINT = IDD × VDD watts (chip internal power)
PI/O = Power dissipation on input and output pins (user-determined)
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For most applications, PI/O « PINT and can be neglected.
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The following is an approximate relationship between PD and TJ (neglecting PI/O):
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A
PD = K ÷ (TJ + 273 °C)
Solving equations (1) and (2) for K gives:
K = PD × (TA + 273 °C) + θJA × (PD)
(2)
2
(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 equations (1) and (2)
iteratively for any value of TA.
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ELECTRICAL SPECIFICATIONS
MC68HC805P18
13-2
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