Datasheet

LTM4628
20
4628fe
For more information www.linear.com/LTM4628
APPLICATIONS INFORMATION
Thermal Considerations and Output Current Derating
The thermal resistances reported in the Pin Configura-
tion section of the data sheet are consistent with those
parameters
defined by JESD 51-12 and are intended for
use with finite element analysis (FEA) software modeling
tools that leverage the outcome of thermal modeling,
simulation, and correlation to hardware evaluation per
-
formed on a µModule package mounted to a hardware
test
board defined by JESD 51-9 (“Test Boards for Area
Array Surface Mount Package Thermal Measurements”).
The motivation for providing these thermal coefficients is
found in JESD 51-12 (“Guidelines for Reporting and Using
Electronic Package Thermal Information”).
Many designers may opt to use laboratory equipment
and a test vehicle such as the demo board to anticipate
the µModule regulator’s thermal performance in their ap
-
plication at
various electrical and environmental operating
conditions
to compliment any FEA activities. Without FEA
software, the thermal resistances reported in the Pin Con-
figuration section
are in-and-of themselves not relevant
to providing guidance of thermal performance; instead,
the derating curves provided later in this data sheet can
be used in a manner that yields insight and guidance per
-
taining to one’s application-usage, and can be adapted to
correlate thermal per
formance to one’s own application.
The
Pin Configuration section gives four thermal coeffi-
cients explicitly
defined in JESD 51-12; these coefficients
are quoted or paraphrased below:
1 θ
JA
, the thermal resistance from junction to ambient, is
the natural convection junction-to-ambient air thermal
resistance measured in a one cubic foot sealed enclo
-
sure. 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
, the thermal resistance from junction to the
bottom of the product case, is determined 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 pack
-
age, 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
, the thermal resistance from junction to top of
the product case, is determined with nearly all of the
component
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
JCbottom
, this value may be useful
for comparing packages but the test conditions don’t
generally match the user’s application.
4 θ
JB
, the thermal resistance from junction to the printed
circuit board, is the junction-to-board thermal resis-
tance where
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
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.
A graphical representation of the aforementioned thermal
resistances is given in Figure 10; blue resistances are
contained within the µModule regulator, whereas green
resistances are external to the µModule package.
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.