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| 型號： | DSP56857E |
| 廠商： | 飛思卡爾半導(dǎo)體(中國)有限公司 |
| 英文描述： | 16-bit Digital Signal Controllers |
| 中文描述： | 16位數(shù)字信號控制器 |
| 文件頁數(shù)： | 48/53頁 |
| 文件大小： | 680K |
| 代理商： | DSP56857E |
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56857 Technical Data, Rev. 6
48
Freescale Semiconductor
Part 6 Design Considerations
6.1 Thermal Design Considerations
An estimation of the chip junction temperature, T
J
, in
°
C can be obtained from the equation:
Equation 1:
T
J
= T
A
+ (P
D
x R
θ
JA
)
Where:
T
A
= ambient temperature °C
R
θ
JA
= package junction-to-ambient thermal resistance °C/W
P
D
= power dissipation in package
Historically, thermal resistance has been expressed as the sum of a junction-to-case thermal resistance and
a case-to-ambient thermal resistance:
Equation 2:
R
θ
JA
= R
θ
JC
+ R
θ
CA
Where:
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
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 example, the user can change the air flow around
the device, add a heat sink, change the mounting arrangement on the Printed Circuit Board (PCB), or
otherwise change the thermal dissipation capability of the area surrounding the device on the PCB. This
model is most useful for ceramic packages with heat sinks; some 90% of the heat flow is dissipated through
the case to the heat sink and out to the ambient environment. For ceramic packages, in situations where
the heat flow is split between a path to the case and an alternate path through the PCB, analysis of the
device thermal performance may need the additional modeling capability of a system level thermal
simulation tool.
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 definitions for determining the junction-to-case
thermal resistance in plastic packages:
Measure the thermal resistance 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. This is done to minimize temperature variation
across the surface.
Measure the thermal resistance from the junction to where the leads are attached to the case. This definition
is approximately equal to a junction to board thermal resistance.
Use the value obtained by the equation (T
J
– T
T
)/P
D
where T
T
is the temperature of the package case
determined by a thermocouple.
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