MC9S08RC8 Datasheet, PDF (206/234 Pages) Freescale Semiconductor, Inc – Microcontrollers

Electrical Characteristics
A.3 Thermal Characteristics
This section provides information about operating temperature range, power dissipation, and package
thermal resistance. Power dissipation on I/O pins is usually small compared to the power dissipation in
on-chip logic and voltage regulator circuits and it is user-determined rather than being controlled by the
MCU design. In order to take P_I/O into account in power calculations, determine the difference between
actual pin voltage and V_SS or V_DD and multiply by the pin current for each I/O pin. Except in cases of
unusually high pin current (heavy loads), the difference between pin voltage and V_SS or V_DD will be very
small.
Table A-2. Thermal Characteristics
Rating
Operating temperature range (packaged)
Thermal resistance
28-pin PDIP
28-pin SOIC
32-pin LQFP
44-pin LQFP
48-pin QFN
Symbol
T_A
θ_JA
Value
T_L to T_H
–40 to 85
75
70
72
70
82
Unit
°C
°C/W
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 equations 1 and 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 equations 1 and 2 iteratively for any value of T_A.
MC9S08RC/RD/RE/RG Data Sheet, Rev. 1.11
206
Freescale Semiconductor