MC68HC705P9 Datasheet, PDF (134/156 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Specifications _{Power Considerations}
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 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 = -_T---_J-------+----^K2----₇---₃-----_°---_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.
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Specifications
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