Power Considerations - Motorola M68000 User Manual

10.3 POWER CONSIDERATIONS

The average die-junction temperature, T J , in C can be obtained from:
T J = T A +(P D • J A )
where:
T A = Ambient Temperature, C
J A = Package Thermal Resistance, Junction-to-Ambient, C/W
P D = P INT + P I/O
P INT = I CC x V CC , 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 appropriate 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:
K = P D • (T A + 273 C) + JA • P D
where K is a constant pertaining to the particular part. K can be determined from equation
(3) by measuring P D (at thermal 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 .
The curve shown in Figure 10-1 gives the graphic solution to the above equations for the
specified power dissipation of 1.5 W over the ambient temperature range of -55 C to 125
C using a maximum J A of 45 C/W. Ambient temperature is that of the still air
surrounding the device. Lower values of J A cause the curve to shift downward slightly; for
instance, for J A of 40 /W, the curve is just below 1.4 W at 25 C.
The total thermal resistance of a package ( J A ) can be separated into two components,
J C and C A , representing the barrier to heat flow from the semiconductor junction to the
package (case) surface ( J C ) and from the case to the outside ambient air ( C A ). These
terms are related by the equation:
J A = J C + C A
J C is device related and cannot be influenced by the user. However, C A is user
dependent and can be minimized by such thermal management techniques as heat sinks,
ambient air cooling, and thermal convection. Thus, good thermal management on the part
of the user can significantly reduce
Substitution of J C for
temperature.
10-2 M68000 8-/16-/32-BIT MICROPROCESSORS USER'S MANUAL
Freescale Semiconductor, Inc.
2
C A so that
J A in equation 1 results in a lower semiconductor junction
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J A approximately equals ; J C .
(1)
(2)
(3)
(4)
MOTOROLA