MC68HC705J2 Datasheet, PDF (68/82 Pages) Freescale Semiconductor, Inc – member of the low-cost

Freescale Semiconductor, Inc.
11.3 Power Considerations
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The average chip-junction temperature, T_J, in °C, can be obtained from:
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T_J = T_A + (P_D × θ_JA)
(1)
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where:
T_A = Ambient temperature, °C
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θ_JA = Package thermal resistance, junction to ambient, °C/W
P_D = P_INT + P_I/O
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P_INT = I_DD × V_DD watts (chip internal power)
P_I/O = Power dissipation on input and output pins (user-determined)
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For most applications P_I/O << P_INT and can be neglected.
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The following is an approximate relationship between P_D and T_J (neglecting P_I/O):
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P_D = K ÷ (T_J + 273 °C)
(2)
Solving equations (1) and (2) for K gives:
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K = P_D × (T_A + 273 °C) + θ_JA × (P_D)²
(3)
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where K is a constant pertaining to the particular part. K can be determined from
equation (3) by measuring P_D (at equilibrium) for a known T_A. Using this value of
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K, the values of P_D and T_J can be obtained by solving equations (1) and (2)
iteratively for any value of T_A.
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11-2
ELECTRICAL SPECIFICATIONS
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Rev. 2