LTM8029_1208 LINER [Linear Technology], LTM8029_1208 Datasheet - Page 15

• Manufacturer Part Number: LTM8029_1208
• Description: 36VIN 600mA Step-Down ?Module Converter with 5?A Quiescent Current
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
Thermal Considerations
The LTM8029 output current may need to be derated if
it is required to operate in a high ambient temperature or
deliver a large amount of continuous power. The amount
of current derating is dependent upon the input voltage,
output power and ambient temperature. The temperature
rise curves given in the Typical Performance Character-
istics section can be used as a guide. These curves were
generated by a LTM8029 mounted to a 40cm
printed circuit board. Boards of other sizes and layer count
can exhibit different thermal behavior, so it is incumbent
upon the user to verify proper operation over the intended
system’s line, load and environmental operating conditions.
The thermal resistance numbers listed in the Pin Con-
figuration are based on modeling the µModule package
mounted on a test board specified per JESD 51-9 (“Test
Boards for Area Array Surface Mount Package Thermal
Measurements”). The thermal coefficients provided in this
page are based on JESD 51-12 (“Guidelines for Reporting
and Using Electronic Package Thermal Information”).
For increased accuracy and fidelity to the actual application,
many designers use FEA to predict thermal performance.
To that end, the Pin Configuration section typically gives
four thermal coefficients:
• θ
• θ
• θ
• θ
bottom of the product case
product case
circuit board
JA
JCbottom
JCtop
JB
– Thermal resistance from junction to ambient
– Thermal resistance from junction to the printed
– Thermal resistance from junction to top of the
– Thermal resistance from junction to the
2
4-layer FR4
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:
• θ
• θ
• θ
• θ
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.
and bottom of the package 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 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.
power dissipation flowing through the top of the pack-
age. 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 junction to the top of the part. As in the
case of θ
packages but the test conditions don’t generally match
the user’s application.
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
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.
JA
JCbottom
JCtop
JB
is the junction-to-board thermal resistance where
is the natural convection junction-to-ambient air
is determined with nearly all of the component
JCbottom
is the thermal resistance between the junction
, this value may be useful for comparing
JCbottom
and the thermal resistance
LTM8029
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