
Electrical Specifications
MC68HC705KJ1 MC68HRC705KJ1 MC68HLC705KJ1 Data Sheet, Rev. 4.1
86
Freescale Semiconductor
10.4 Power Considerations
The average chip junction temperature, T
J
, in
°
C can be obtained from:
T J
(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:
(2)
Solving equations (1) and (2) for K gives:
(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
.
T A
P D
θ
×
JA
(
)
+
=
P D
T J
273
°
C
+
-----------------------------------
=
= P
D
x (T
A
+ 273°C) +
Θ
J
A
x (P
D
)