MC68HC901 Datasheet, PDF (55/73 Pages) Motorola, Inc – Multi-Function Peripheral

Electrical Characteristics
8.3 POWER CONSIDERATIONS
The average chip-junction temperature, T_J, in ˚C can be obtained from:
T_J = T_A + (P_D • θ_JA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (1)
where:
T_A
θ_JA
P_D
P_INT
P_I/O
= Ambient Temperature, ˚C
= Package Thermal Resistance, Junction-to-Ambient, ˚C/W
= P_INT + P_I/O
= I_CC x V_CC, Watts — Chip Internal Power
= Power Dissipation on Input and Output Pins, Watts — 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) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2)
Solving equations (1) and (2) for K gives:
K = P_D • (T_A + 273 ˚C) + θ_JA • P_D2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (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.
The total thermal resistance of a package (θ_JA) 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 (θ_JC) and from the case to the outside ambient (θ_CA). 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 θ_CA so that θ_JA approximately equals θ_JC. Substitution of θ_JC for θ_JA
in equation (1) will result in a lower semiconductor junction temperature.
8-2
MC68HC901 USER’S MANUAL
MOTOROLA