LTM4622A
15
Rev B
APPLICATIONS INFORMATION
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 JESD51-12; these coef-
ficients are quoted or paraphrased below:
1.
θ
JA
, the thermal resistance from junction to ambi-
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
JCbottom
, the thermal resistance from junction to
ambient, 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.
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 re-
sistance 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 ther-
mal resistances is given in Figure 7; blue resistances are
contained within the μModule regulator, whereas green
resistances are external to the µModule.
Figure 7. Graphical Representation of JESD51-12 Thermal Coefficients
4622A 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
AMBIENT
CASE (BOTTOM)-TO-BOARD
RESISTANCE