MC68HC705C9A_1 Datasheet, PDF (132/157 Pages) Freescale Semiconductor, Inc – General Release Specification

Freescale Semiconductor, Inc.
General Release Specification _{Power Considerations}
12.5 Power Considerations
The average chip-junction temperature, T_J, in °C, can be obtained from:
T_J = T_A + (P_D × θ_JA)
(1)
where:
T_A = Ambient temperature, °C
θ_JA = Package thermal resistance, junction to ambient, °C/W
P_D = P_INT + P_I/O
P_INT = I_DD × V_DD watts (chip internal power)
P_I/O = Power dissipation on input and output pins (user determined)
For most applications P_I/O « P_INT and can be neglected.
The following is an approximate relationship between P_D and T_J
(neglecting P_J):
P_D = K ÷ (T_J + 273 °C)
(2)
Solving equations (1) and (2) for K gives:
K = P_D × (T_A + 273 °C) + θ_JA × (P_D)²
(3)
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 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.
MC68HC705C9A — Rev. 2.0
Electrical Specifications
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