LTM8047
13
8047fc
For more information www.linear.com/LTM8047
APPLICATIONS INFORMATION
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
JB
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 3.
The blue resistances are contained within the µModule
converter, and the green are outside.
The die temperature of the LTM8047 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
LTM8047. The bulk of the heat flow out of the LTM8047
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 cir
cuit board design
suggestions.
Figure 3.
8047 F03
µMODULE DEVICE
JUNCTION-TO-CASE (TOP)
RESISTANCE
JUNCTION-TO-BOARD RESISTANCE
JUNCTION-TO-AMBIENT RESISTANCE (JESD 51-9 DEFINED BOARD)
CASE (TOP)-TO-AMBIENT
RESISTANCE
BOARD-TO-AMBIENT
RESISTANCE
JUNCTION-TO-CASE
(BOTTOM) RESISTANCE
JUNCTION
CASE (BOTTOM)-TO-BOARD
RESISTANCE
