參數(shù)資料
型號: MC56F8345
廠商: Motorola, Inc.
英文描述: 56F8345 16-bit Hybrid Controller
中文描述: 56F8345 16位混合控制器
文件頁數(shù): 144/148頁
文件大小: 1420K
代理商: MC56F8345
144
56F8345 Technical Data
Preliminary
Part 12 Design Considerations
12.1 Thermal Design Considerations
An estimation of the chip junction temperature, T
J
, can be obtained from the equation:
T
J
= T
A
+ (R
θ
J
Α
x
P
D
)
where:
= Ambient temperature for the package (
o
C)
R
θ
J
Α
= Junction-to-ambient thermal resistance (
o
C/W)
P
D
= Power dissipation in the package (W)
The junction-to-ambient thermal resistance is an industry-standard value that provides a quick and
easy estimation of thermal performance. Unfortunately, there are two values in common usage: the
value determined on a single-layer board and the value obtained on a board with two planes. For
packages such as the PBGA, these values can be different by a factor of two. Which value is closer
to the application depends on the power dissipated by other components on the board. The value
obtained on a single-layer board is appropriate for the tightly packed printed circuit board. The
value obtained on the board with the internal planes is usually appropriate if the board has
low-power dissipation and the components are well separated.
When a heat sink is used, the thermal resistance is expressed as the sum of a junction-to-case
thermal resistance and a case-to-ambient thermal resistance:
R
θ
JA
= R
θ
JC
+ R
θ
CA
where:
R
θ
JC
is device-related and cannot be influenced by the user. The user controls the thermal
environment to change the case-to-ambient thermal resistance, R
θ
CA
. For instance, the user can
change the size of the heat sink, the air flow around the device, the interface material, the mounting
arrangement on printed circuit board, or change the thermal dissipation on the printed circuit board
surrounding the device.
To determine the junction temperature of the device in the application when heat sinks are not used,
the Thermal Characterization Parameter (
Ψ
JT
) can be used to determine the junction temperature
with a measurement of the temperature at the top center of the package case using the following
equation:
T
J
= T
T
+ (
Ψ
JT
x P
D
)
where:
= Thermocouple temperature on top of package (
o
C)
Ψ
JT
P
D
= Power dissipation in package (W)
T
A
R
θ
JA
= Package junction-to-ambient thermal resistance °C/W
R
θ
JC
= Package junction-to-case thermal resistance °C/W
R
θ
CA
= Package case-to-ambient thermal resistance °C/W
T
T
= Thermal characterization parameter (
o
C)/W
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
.
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