LTM8029 Datasheet, PDF (15/20 Pages) Linear Technology – 36VIN, 600mA Step-Down μModule Converter

LTM8029
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 40cm² 4-layer FR4
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_JA – Thermal resistance from junction to ambient
• θ_JCbottom – Thermal resistance from junction to the
bottom of the product case
• θ_JCtop – Thermal resistance from junction to top of the
product case
• 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:
• 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 thermal resistance between the junction
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.
• 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
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_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.
8029f
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