LTM8047 LINER [Linear Technology], LTM8047 Datasheet - Page 15

• Manufacturer Part Number: LTM8047
• Description: 3.1VIN to 32VIN Isolated ?Module DC/DC Converter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
θ
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
JESD 51-9 defined test board, which does not reflect an
actual application or viable operating condition.
θ
all of the component power dissipation flowing through the
bottom of the package. In the typical μModule converter,
the bulk of the heat flows out the bottom of the package,
but there is always heat flow out into the ambient envi-
ronment. 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.
θ
dissipation flowing through the top of the package. As the
electrical connections of the typical μModule converter are
on the bottom of the package, it is rare for an application
to operate such that most of the heat flows from the junc-
tion to the top of the part. As in the case of θ
value may be useful for comparing packages but the test
conditions don’t generally match the user’s application.
θ
almost all of the heat flows through the bottom of the
μModule converter and into the board, and is really the
sum of the θ
bottom of the part through the solder joints and through a
JA
JCbottom
JCtop
JCboard
is the natural convection junction-to-ambient air
is determined with nearly all of the component power
is the junction-to-board thermal resistance where
is the junction-to-board thermal resistance with
JCbottom
JUNCTION
μMODULE DEVICE
and the thermal resistance of the
(BOTTOM) RESISTANCE
JUNCTION-TO-CASE
JUNCTION-TO-AMBIENT RESISTANCE (JESD 51-9 DEFINED BOARD)
JUNCTION-TO-BOARD RESISTANCE
JUNCTION-TO-CASE (TOP)
JCbottom
RESISTANCE
CASE (BOTTOM)-TO-BOARD
, this
RESISTANCE
Figure 2.
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 JESD 51-9.
Given these definitions, it should now be apparent that none
of these thermal coefficients reflects an actual physical
operating condition of a μModule converter. Thus, none
of them can be individually used to accurately predict the
thermal performance of the product. Likewise, it would
be inappropriate to attempt to use any one coefficient to
correlate to the junction temperature vs load graphs given
in the product’s data sheet. The only appropriate way to
use the coefficients is when running a detailed thermal
analysis, such as FEA, which considers all of the thermal
resistances simultaneously.
A graphical representation of these thermal resistances
is given in Figure 2.
The blue resistances are contained within the μModule
converter, and the green are outside.
The die temperature of the LTM8048 must be lower than
the maximum rating of 125°C, so care should be taken in
the layout of the circuit to ensure good heat sinking of the
LTM8048. The bulk of the heat flow out of the LTM8048
is through the bottom of the module and the BGA pads
into the printed circuit board. Consequently a poor printed
circuit board design can cause excessive heating, result-
ing in impaired performance or reliability. Please refer to
the PCB Layout section for printed circuit board design
suggestions.
CASE (TOP)-TO-AMBIENT
BOARD-TO-AMBIENT
RESISTANCE
RESISTANCE
8048 F02
AMBIENT
LTM8048
15
8048fa