MC68HC05C9A Datasheet, PDF (89/124 Pages) Freescale Semiconductor, Inc – Microcontrollers

Power Considerations
12.4 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.
MC68HC05C9A Advance Information Data Sheet, Rev. 5.1
Freescale Semiconductor
89