Техническая Спецификация для Freescale Semiconductor MC56F8006 Demo board MC56F8006DEMO MC56F8006DEMO
Модели
MC56F8006DEMO
MC56F8006/MC56F8002 Digital Signal Controller, Rev. 4
Design Considerations
Freescale Semiconductor
70
9
Design Considerations
9.1
Thermal Design Considerations
An estimation of the chip junction temperature, T
J
, can be obtained from the equation:
T
J
= T
A
+ (R
J
x P
D
)
Eqn. 3
where:
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.
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:
case-to-ambient thermal resistance:
R
JA
= R
JC
+ R
CA
Eqn. 4
where:
R
JC
is device related and cannot be adjusted. You control the thermal environment to change the case to ambient thermal
resistance, R
CA
. For instance, you 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.
the device.
To determine the junction temperature of the device in the application when heat sinks are not used, the thermal characterization
parameter (
parameter (
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
+ (
JT
x P
D
)
Eqn. 5
where:
The thermal characterization parameter is measured per JESD51–2 specification using a 40-gauge type T thermocouple epoxied
to the top center of the package case. The thermocouple should be positioned so that the thermocouple junction rests on the
package. A small amount of epoxy is placed over the thermocouple junction and over about 1 mm of wire extending from the
to the top center of the package case. The thermocouple should be positioned so that the thermocouple junction rests on the
package. A small amount of epoxy is placed over the thermocouple junction and over about 1 mm of wire extending from the
T
A
=
Ambient temperature for the package (
o
C)
R
J
=
Junction-to-ambient thermal resistance (
o
C/W)
P
D
=
Power dissipation in the package (W)
R
JA
=
Package junction-to-ambient thermal resistance (°C/W)
R
JC
=
Package junction-to-case thermal resistance (°C/W)
R
CA
=
Package case-to-ambient thermal resistance (°C/W)
T
T
=
Thermocouple temperature on top of package (
o
C)
JT
=
Thermal characterization parameter (
o
C/W)
P
D
=
Power dissipation in package (W)