參數(shù)資料
型號(hào)： MC68176
廠商： Motorola, Inc.
英文描述： 2-bit A/D Converter(2位A/D轉(zhuǎn)換器)
中文描述： 2位A / D轉(zhuǎn)換器（2位的A / D轉(zhuǎn)換器）
文件頁(yè)數(shù)： 20/29頁(yè)
文件大小： 235K
代理商： MC68176
Design Considerations
Thermal Design Considerations
Preliminary
4-2
MC68176 Technical Data Sheet
MOTOROLA
The thermal performance of plastic packages is more dependent on the temperature
of the PCB to which the package is mounted. Again, if the estimations obtained from
R
θ
JA
do not satisfactorily answer whether the thermal performance is adequate, a
system level model may be appropriate.
A complicating factor is the existence of three common ways for determining the
junction-to-case thermal resistance in plastic packages:
To minimize temperature variation across the surface, the thermal resistance
is measured from the junction to the outside surface of the package (case)
closest to the chip mounting area when that surface has a proper heat sink.
To define a value approximately equal to a junction-to-board thermal
resistance, the thermal resistance is measured from the junction to where the
leads are attached to the case.
If the temperature of the package case (T
T
) is determined by a thermocouple,
the thermal resistance is computed using the value obtained by the equation
(T
J
T
T
) / P
D
.
As noted above, the junction-to-case thermal resistances quoted in this data sheet are
determined using the first definition. From a practical standpoint, that value is also
suitable for determining the junction temperature from a case thermocouple reading
in forced convection environments. In natural convection, using the junction-to-case
thermal resistance to estimate junction temperature from a thermocouple reading on
the case of the package will estimate a junction temperature slightly hotter than actual
temperature. Hence, the new thermal metric, Thermal Characterization Parameter or
Ψ
JT
, has been defined to be (T
J
T
T
) / P
D
. This value gives a better estimate of the
junction temperature in natural convection when using the surface temperature of the
package. Remember that surface temperature readings of packages are subject to
significant errors caused by inadequate attachment of the sensor to the surface and to
errors caused by heat loss to the sensor. The recommended technique is to attach a 40-
gauge thermocouple wire and bead to the top center of the package with thermally
conductive epoxy.
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