Datasheet

LTM4613
17
4613f
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 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 guid-
ance pertaining to one’s application usage, and can be
adapted to correlate thermal performance to one’s own
application.
The Pin Configuration section of the data sheet typi-
cally gives four thermal coefficients, explicitly defined in
JESD51-12. These coefficients are quoted or paraphrased
below:
• 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 JESD51-9 defined test board, which does not reflect
an actual application or viable operating condition.
• θ
JCbottom
, the thermal resistance from the junction to
the bottom of the product case, is the junction-to-board
thermal resistance with all of the component power
dissipation flowing through the bottom of the pack-
age. In the typical µModule regulator, the bulk of the
heat flows out of 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 do not
generally match the user’s application.
• θ
JCtop
, the thermal resistance from the junction to the
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
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 do not
generally match the user’s application.
• θ
JB
, the thermal resistance from the 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 regulator and into the board.
It 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 JESD51-9.
Figure 8. Graphical Representation of JESD51-12 Thermal Coefficients
APPLICATIONS INFORMATION
µModule REGULATOR
4613 F08
JUNCTION A
t
JUNCTION-TO-CASE (TOP)
RESISTANCE
JUNCTION-TO-AMBIENT RESISTANCE (JESD51-9 DEFINED BOARD)
JUNCTION-TO-BOARD RESISTANCE
CASE (TOP)-TO-AMBIENT
RESISTANCE
BOARD-TO-AMBIENT
RESISTANCE
CASE (BOTTOM)-TO-BOARD
RESISTANCE
JUNCTION-TO-CASE (BOTTOM)
RESISTANCE