Analog Devices Linear Technology LTM8026 Manual page 19

APPLICATIONS INFORMATION
θ is the junction-to-board thermal resistance with
JCbottom
all of the component power dissipation flowing through the
bottom of the package. In the typical µModule regulator,
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 is determined with nearly all of the component power
JCtop
dissipation flowing through the top of the package. As the
electrical connections of the typical µModule regulator 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
value may be useful for comparing packages but the test
conditions don't generally match the user's application.
θ is the junction-to-board thermal resistance where
JB
almost all of the heat flows through the bottom of the
µModule regulator and into the board, and is really the
sum of the θ and the thermal resistance of the
JCbottom
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 2-sided,
2-layer board. This board is described in JESD 51-9.
JUNCTION
µMODULE DEVICE
, this
JCbottom
JUNCTION-TO-AMBIENT RESISTANCE (JESD 51-9 DEFINED BOARD)
JUNCTION-TO-CASE (TOP)
RESISTANCE
JUNCTION-TO-BOARD RESISTANCE
JUNCTION-TO-CASE CASE (BOTTOM)-TO-BOARD
(BOTTOM) RESISTANCE RESISTANCE
Figure 6
For more information
Given these definitions, it should now be apparent that none
of these thermal coefficients reflects an actual physical
operating condition of a µModule regulator. 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 6.
The blue resistances are contained within the µModule
device, and the green are outside.
The die temperature of the LTM8026 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
LTM8026. The bulk of the heat flow out of the LTM8026
is through the bottom of the module and the 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.
CASE (TOP)-TO-AMBIENT
RESISTANCE
BOARD-TO-AMBIENT
RESISTANCE
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LTM8026
AMBIENT
8026 F06
19
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