MC68HC705C8ACFNE Datasheet, PDF (174/222 Pages) Freescale Semiconductor, Inc – Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.
Electrical Specifications
13.6 Power Considerations
The average chip junction temperature, T_J, in °C can be obtained from:
T_J = T_A + (P_D x θ_JA)
(1)
Where:
T_A = ambient temperature in °C
θ_JA = package thermal resistance, junction to ambient in °C/W
P_D = P_INT + P_I/O
P_INT = I_CC × V_CC = chip internal power dissipation
P_I/O = power dissipation on input and output pins (user-determined)
For most applications, P_I/O < P_INT and can be neglected.
Ignoring P_I/O, the relationship between P_D and T_J is approximately:
P_D =
K
T_J + 273°C
(2)
Solving equations (1) and (2) for K gives:
= P_D x (T_A + 273°C) + θ_JA x (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.
Technical Data
174
Electrical Specifications
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MC68HC705C8A — Rev. 3