LTM4641
APPLICATIONS INFORMATION—THERMAL CONSIDERATIONS AND
OUTPUT CURRENT DERATING
Therꢃal Considerations and Output Current Derating
2
θ
, the thermal resistance from junction to the
JCbottom
bottom of the product case, is determined with all of
the component power dissipation flowing through the
bottomofthepackage.InthetypicalµModuleregulator,
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.
The thermal resistances reported in the Pin Configuration
section of the data sheet are consistent with those param-
eters defined by JESD51-12 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 performed on a
µModule package mounted to a hardware test board de-
fined by JESD51-9 (“Test Boards for Area Array Surface
MountPackageThermalMeasurements”).Themotivation
for providing these thermal coefficients is found in JESD
51-12 (“Guidelines for Reporting and Using Electronic
Package Thermal Information”).
3
θ
, the thermal resistance from junction to top of
JCtop
the product case, is determined with nearly all of the
componentpowerdissipationflowingthroughthetopof
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.
Manydesignersmayopttouselaboratoryequipmentanda
testvehiclesuchasthedemoboardtopredicttheµModule
regulator’s thermal performance in their application at
various electrical and environmental operating conditions
to compliment any FEA activities. Without FEA software,
the thermal resistances reported in the Pin Configuration
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 pertaining to
one’s application-usage, and can be adapted to correlate
thermal performance to one’s own application.
As in the case of θ
for comparing packages but the test conditions don’t
generally match the user’s application.
, this value may be useful
JCbottom
4
θ , the thermal resistance from junction to the printed
JB
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
and the thermal
JCbottom
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.
The Pin Configuration section gives four thermal coeffi-
cients explicitly defined in JESD 51-12; these coefficients
are quoted or paraphrased below:
1
θ , the thermal resistance from junction to ambient, is
JA
the natural convection junction-to-ambient air thermal
resistance measured in a one cubic foot sealed enclo-
sure.Thisenvironmentissometimesreferredtoas“still
air”althoughnaturalconvectioncausestheairtomove.
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.
A graphical representation of the aforementioned thermal
resistances is given in Figure 17; blue resistances are
contained within the µModule regulator, whereas green
resistances are external to the µModule package.
As a practical matter, it should be clear to the reader that
no individual or sub-group of the four thermal resistance
4641f
38
Linear [ Linear ]
