10-2
M68000 8-/16-/32-BIT MICROPROCESSORS USER'S MANUAL
MOTOROLA
10.3 POWER CONSIDERATIONS
The average die-junction temperature, TJ, in °C can be obtained from:
TJ = TA+(PD θJA)
(1)
where:
TA
= Ambient Temperature,
°C
θJA
= Package Thermal Resistance, Junction-to-Ambient,
°C/W
PD
= PINT + PI/O
PINT
= ICC x VCC, Watts — Chip Internal Power
PI/O
= Power Dissipation on Input and Output Pins — User Determined
For most applications, PI/O<PINT and can be neglected.
An appropriate relationship between P D and TJ (if PI/O is neglected) is:
PD = K÷(TJ + 273 °C)
(2)
Solving Equations (1) and (2) for K gives:
K = PD (TA + 273°C) + θJA PD2
(3)
where K is a constant pertaining to the particular part. K can be determined from equation
(3) by measuring PD (at thermal equilibrium) for a known TA. Using this value of K, the
values of PD and TJ can be obtained by solving Equations (1) and (2) iteratively for any
value of TA.
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 θ JA of 45 °C/W. Ambient temperature is that of the still air
surrounding the device. Lower values of θJA cause the curve to shift downward slightly; for
instance, for θJA of 40 °/W, the curve is just below 1.4 W at 25 °C.
The total thermal resistance of a package (θJA) can be separated into two components,
θJC and θCA, representing the barrier to heat flow from the semiconductor junction to the
package (case) surface (θJC) and from the case to the outside ambient air (θCA). These
terms are related by the equation:
θJA = θJC + θCA
(4)
θJC is device related and cannot be influenced by the user. However, θCA 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 θCA so that θ JA approximately equals ;θJC.
Substitution of θJC for θ JA in equation 1 results in a lower semiconductor junction
temperature.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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