LTM4622
15
Rev F
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APPLICATIONS INFORMATION
board—also defined by JESD51-9 (Test Boards for Area
Array Surface Mount Package Thermal Measurements).
The motivation for providing these thermal coefficients in
found in JESD51-12 (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 appli-
cation 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 in the 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 typically gives four thermal
coefficients explicitly defined in JESD 51-12; these coef-
ficients are quoted or paraphrased below:
1.
θ
JA
, the thermal resistance from junction to ambient,
is the natural convection junction-to-ambient air ther-
mal 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 operat-
ing condition.
2. θ
JCbottom
, the thermal resistance from junction to
ambient, is the natural convection junction-to-ambi-
ent 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.
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 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
resistance where almost all of the heat flows through
the bottom of the µModule 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.
Figure7. Graphical Representation of JESD 51-12 Thermal Coefficients
4622 F07
µMODULE DEVICE
JUNCTION-TO-CASE (TOP)
RESISTANCE
JUNCTION-TO-BOARD RESISTANCE
JUNCTION-TO-AMBIENT THERMAL RESISTANCE COMPONENTS
CASE (TOP)-TO-AMBIENT
RESISTANCE
BOARD-TO-AMBIENT
RESISTANCE
JUNCTION-TO-CASE
(BOTTOM) RESISTANCE
JUNCTION
CASE (BOTTOM)-TO-BOARD
RESISTANCE
