Datasheet

LTM8062/LTM8062A
18
8062fd
For more information www.linear.com/LTM8062
80421 F08
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 AMBIENT
CASE (BOTTOM)-TO-BOARD
RESISTANCE
1. θ
JA
: Thermal resistance from junction to ambient.
2. θ
JCbottom
: Thermal resistance from junction to the bot-
tom of the product case.
3. θ
JCtop
: Thermal resistance from junction to top of the
product case.
4. θ
JB
: 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 confusion
and inconsistency. These definitions are given in JESD
51-12, and are quoted or paraphrased below:
1. 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.
2. 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 device, the bulk of the heat flows out the bot-
tom of the package, but there is always heat flow out
into the ambient environment. 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.
3. θ
JCtop
is determined with nearly all of the component
power dissipation flowing through the top of the pack-
age. As the electrical connections of the typical µModule
device are on the bottom of the package, it is rare for an
application to operate such that most of the heat flows
from the junction 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.
4. θ
JB
is the junction-to-board thermal resistance where
almost all of the heat flows through the bottom of the
µModule device 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 mea-
sured a specified distance from the package, using a
two sided, two layer board. This board is described in
JESD 51-9.
The most appropriate way to use the coefficients is when
running a detailed thermal analysis, such as FEA, which
considers all of the thermal resistances simultaneously.
None of them can be individually used to accurately pre-
dict the thermal performance of the product, so it would
be inappropriate to attempt to use any one coefficient to
correlate to the junction temperature versus load graphs
given in the Typical Performance Characteristics.
A graphical representation of these thermal resistances
is given in Figure 8.
Figure 8. Thermal Resistances Among µModule Device Printed Circuit Board and Ambient Environment
applicaTions inForMaTion