32
MMC2080/2075 Technical Data
Preliminary
Heat Dissipation
Part 5 Design Considerations
5.1 Heat Dissipation
An estimate of the MMC2080/2075 chip junction temperature, T
J
, in
°
C can be obtained from the following
equation.
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, as follows:
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, or otherwise
change the thermal dissipation capability of the area surrounding the device on a printed circuit board. This
model is most useful for ceramic packages with heat sinks; ninety percent 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 printed circuit board,
analysis of the device
’
s 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 printed circuit
board 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 previously, the junction-to-case thermal resistances quoted in this document are determined using
the first definition. From a practical standpoint, this value is also suitable for determining the junction
T
J
T
A
P
D
R
θ
JA
×
(
)
+
=
R
θ
JA
R
θ
JC
R
θ
CA
+
=