ltm8042 Linear Technology Corporation, ltm8042 Datasheet - Page 26

• Manufacturer Part Number: ltm8042
• Description: Ltm8042/ltm8042-1 - ?module Boost Led Driver And Current Source
• Manufacturer: Linear Technology Corporation

LTM8042/LTM8042-1
APPLICATIONS INFORMATION
Thermal Considerations
The LTM8042/LTM8042-1 output current may need to be
derated if it is required to operate in a high ambient tem-
perature 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
Characteristics section can be used as a guide. These curves
were generated by an LTM8042/LTM8042-1 mounted to a
51cm
sizes and layer count can exhibit different thermal behavior,
so it is in-cumbent 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 Configura-
tion section of the data sheet are based on modeling the
μModule package mounted on a test board specified per
JESD51-9 (“Test Boards for Area Array Surface Mount
Package Thermal Measurements”). The thermal coef-
ficients provided are based on JESD 51-12 (“Guidelines
for Reporting and Using Electronic Package Thermal
Information”).
For increased accuracy and fidelity to the actual applica-
tion, many designers use finite element analysis (FEA) to
predict thermal performance. To that end, the Pin Con-
figuration section of the data sheet typically gives four
thermal coefficients:
1. θ
2. θ
3. θ
4. θ
26
bottom of the product case.
product case.
circuit board.
JA
JCBOTTOM
JCTOP
JB
2
: thermal resistance from junction to the printed
: thermal resistance from junction to ambient.
4-layer FR4 printed circuit board. Boards of other
: thermal resistance from junction to top of the
: thermal resistance from junction to the
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 in the following:
1. θ
2. θ
3. θ
4. 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.
with 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 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
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 junction to the top of the part.
As in the case of θ
for comparing 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 regulator and into the board, and is really the
sum of the θ
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.
JCBOTTOM
JCTOP
JB
JA
is the junction-to-board thermal resistance where
is the natural convection junction-to-ambient air
is determined with nearly all of the component
is the junction-to-board thermal resistance
JCBOTTOM
JCBOTTOM
and the thermal resistance of
, this value may be useful
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