MC68HC000RC12 Datasheet, PDF (142/189 Pages) Freescale Semiconductor, Inc – 8-/16-/32-Bit Microprocessors User’s Manual

Freescale Semiconductor, Inc.
10.3 POWER CONSIDERATIONS
The average die-junction temperature, TJ, in °C can be obtained from:
TJ = TA+(PD • ^θJA)
(1)
where:
TA
θ_JA
PD
PINT
PI/O
= Ambient Temperature, °C
= Package Thermal Resistance, Junction-to-Ambient, °C/W
= PINT + PI/O
= ICC _x VCC, Watts — Chip Internal Power
= Power Dissipation on Input and Output Pins — User Determined
For most applications, PI/O<PINT and can be neglected.
An appropriate relationship between PD 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,
θ_{JC and} θ_{CA, 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:
θ_{JA =} θ_{JC +} θ_CA
(4)
θ_{JC is device related and cannot be influenced by the user. However,} θ _{CA 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.
10-2
M68000 8-/16-/32-BIT MICROPROCESSORS USER'S MANUAL
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