LTM4636
20
4636f
applicaTions inForMaTion
Many designers may opt to use laboratory equipment and a
test vehicle such as the demo board to predict the µModule
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 in this data sheet can be used in a man-
ner that yields insight and guidance pertaining to one’s
application usage, and can be adapted to correlate thermal
performance to one’s own application.
The Pin Configuration section gives four thermal coeffi-
cients explicitly defined in JESD51-12; 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 95mm
×
76mm PCB with four layers.
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 resistance
where almost all of the heat flows through the bottom
of the µModule package 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 a
portion of the board. The board temperature is measured
a specified distance from the package.
A graphical representation of the aforementioned ther-
mal resistances is given in Figure 9; blue resistances are
contained within the µModule regulator, whereas green
resistances are external to the µModule package.
Figure 9. Graphical Representation of JESD51-12 Thermal Coefficients
4637 F09
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
At
CASE (BOTTOM)-TO-BOARD
RESISTANCE
