MC68HC705KJ1 Datasheet, PDF (86/108 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Electrical Specifications
10.4 Power Considerations
The average chip junction temperature, T_J, in °C can be obtained from:
TJ = TA + (PD × 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:
PD = _T----_J--------+----^K----₂---₇---₃-----_°--_C--
(2)
Solving equations (1) and (2) for K gives:
= P_D x (T_A + 273°C) + ΘJ_A x (P_D)
(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.
MC68HC705KJ1 • MC68HRC705KJ1 • MC68HLC705KJ1 Data Sheet, Rev. 4.1
86
Freescale Semiconductor