Datasheet

LTM4620A
22
4620afb
For more information www.linear.com/LTM4620A
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 in the 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.
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 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 the junction-to-board
thermal resistance with all of the component power
dissipation flowing through the bottom of the package.
In the typical µ
Module, the bulk of the heat flows out
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 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
APPLICATIONS INFORMATION
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 por-
tion
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 ther
-
mal resistances is given in Figure 9; blue resistances are
contained within the µModule regulator, whereas green
resistances are external to the µModule.
As a practical matter, it should be clear to the reader that
no individual or sub-group of the four thermal resistance
parameters defined by JESD 51-12 or provided in the
Pin Configuration section replicates or conveys normal
operating conditions of a µModule. For example, in normal
board-mounted applications, never does 100% of the
device’s total power loss (heat) thermally conduct exclu
-
sively through
the top or exclusively through bottom of the
µModule—as the standard defines for θ
JCtop
and θ
JCbottom
,
respectively. In practice, power loss is thermally dissipated
in both directions away from the package—granted, in the
absence of a heat sink and airflow, a majority of the heat
flow is into the board.