Analog Devices Linear LTM 4700 User Manual page 63

APPLICATIONS INFORMATION
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 Con-
figuration 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 per-
taining 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 , the thermal resistance from junction to ambi-
JA
ent, is the natural convection junction-to-ambient
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 JESD51-9 defined test board,
which does not reflect an actual application or viable
operating condition.
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 bottom of the package. In the typical µModule
regulator, 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
JUNCTION
µModule DEVICE
JUNCTION-TO-AMBIENT THERMAL RESISTANCE COMPONENTS
JUNCTION-TO-CASE (TOP)
RESISTANCE
JUNCTION-TO-BOARD RESISTANCE
JUNCTION-TO-CASE CASE (BOTTOM)-TO-BOARD
(BOTTOM) RESISTANCE
Figure 34. Graphical Representation of JESD51-12 Thermal Coefficients
For more information
resistance value may be useful for comparing pack-
ages but the test conditions don't generally match the
user's application.
3. θ , the thermal resistance from junction to top of
JCtop
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
may be useful for comparing packages but the test
conditions don't generally match the user's application.
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
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.
A graphical representation of the aforementioned thermal
resistances is given in Figure 34; 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
CASE (TOP)-TO-AMBIENT
RESISTANCE
BOARD-TO-AMBIENT
RESISTANCE RESISTANCE
www.analog.com
LTM4700
, this value
JCbottom
and the thermal
JCbottom
AMBIENT
4700 F34
63
Rev. B