LTM8058
15
8058fa
For more information www.linear.com/LTM8058
APPLICATIONS INFORMATION
it is incumbent upon the user to verify proper operation
over the intended system’s line, load and environmental
operating conditions.
For increased accuracy and fidelity to the actual application,
many designers use FEA to predict thermal performance.
To that end, the Pin Configuration section of the data sheet
typically gives four thermal coefficients:
θ
JA
: Thermal resistance from junction to ambient
θ
JCbottom
: Thermal resistance from junction to the bot-
tom of the product case
θ
JCtop
: Thermal resistance from junction to top of the
product case
θ
JCboard
: Thermal resistance from junction to the printed
circuit board.
While the meaning of each of these coefficients may seem
to be intuitive, JEDEC has defined each to avoid confu
-
sion and inconsistency. These definitions are given in
JESD 51-12, and are quoted or paraphrased as follows:
θ
JA
is the natural convection junction-to-ambient air
thermal resistance measured in a one cubic foot sealed
enclosure. This environment is sometimes referred to
as still air although natural convection causes the air to
move. This value is determined with the part mounted to a
JESD 51-9 defined
test board, which does not reflect an
actual application or viable operating condition.
θ
JCbottom
is the junction-to-board thermal resistance with
all of the component power dissipation flowing through the
bottom of the package. In the typical µModule converter,
the bulk of the heat flows out the bottom of the package,
but there is always heat flow out into the ambient envi
-
ronment. As
a result, this thermal resistance value may
be
useful for comparing packages but the test conditions
don’t generally match the user’s application.
θ
JCtop
is determined with nearly all of the component power
dissipation flowing through the top of the package. As the
electrical connections of the typical µModule converter are
on the bottom of the package, it is rare for an application
to operate such that most of the heat flows from the junc
-
tion to the top of the part. As in the case of θ
JCbottom
, this
value may be useful for comparing packages but the test
conditions don’t generally match the user’s application.
θ
JCboard
is the junction-to-board thermal resistance where
almost all of the heat flows through the bottom of the
µModule converter and into the board, and is really the
sum of the θ
JCbottom
and the thermal resistance of the
bottom of the part through the solder joints and through a
portion
of the board. The board temperature is measured
a specified distance from the package, using a two-sided,
two-layer board. This board is described in JESD 51-9.
Given these definitions, it should now be apparent that none
of these thermal coefficients reflects an actual physical
operating condition of a µModule converter. Thus, none
of them can be individually used to accurately predict the
thermal performance of the product. Likewise, it would
be inappropriate to attempt to use any one coefficient to
correlate to the junction temperature vs load graphs given
in the product’s data sheet. The only appropriate way to
use the coefficients is when running a detailed thermal
analysis, such as FEA, which considers all of the thermal
resistances simultaneously.
A graphical representation of these thermal resistances
is given in Figure 4.
The blue resistances are contained within the µModule
converter, and the green are outside.
The die temperature of the LTM8058 must be lower than
the maximum rating of 125°C, so care should be taken in
the layout of the circuit to ensure good heat sinking of the
LTM8058. The bulk of the heat flow out
of the LTM8058
is
through the bottom of the module and the BGA pads
into the printed circuit board. Consequently a poor printed
circuit board design can cause excessive heating, result
-
ing in impaired performance or reliability. Please refer to
the
PCB Layout section for printed circuit board design
suggestions.
