MC9S08QG8 Datasheet, PDF (259/300 Pages) Freescale Semiconductor, Inc – 8-BIT HCS08 Central Processor Unit

Appendix A Electrical Characteristics
The average chip-junction temperature (T_J) in °C can be obtained from:
where:
T_J = T_A + (P_D × θ_JA)
Eqn. A-1
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. An approximate relationship between P_D and T_J
(if P_I/O is neglected) is:
P_D = K ÷ (T_J + 273°C)
Solving Equation A-1 and Equation A-2 for K gives:
Eqn. A-2
K = P_D × (T_A + 273°C) + θ_JA × (P_D)²
Eqn. A-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 Equation A-1 and Equation A-2 iteratively for any value of T_A.
MC9S08QG8 and MC9S08QG4 Data Sheet, Rev. 1.01
Freescale Semiconductor
259