Datasheet

LTM4611
18
4611fb
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applicaTions inForMaTion
4. θ
JB
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.
Given these definitions, it should now be apparent that
none of these thermal coefficients reflects an actual
physical operating condition of a µModule. 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 versus load graphs
given in the product’s data sheet. The only appropriate
way to use the coefficients is to run 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 6.
The blue resistances are contained within the µModule,
and the green are outside.
The die temperature of the LTM4611 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
LTM4611. The bulk of the heat flow out of the LTM4611 is
through the bottom of the module and the LGA 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
The 1.2V, 2.5V and 3.3V power loss curves in Figures 7
and 8 can be used in coordination with the load current
derating curves in Figures 9 to 16 for calculating an
approximate θ
JA
thermal resistance for the LTM4611
with various heat sinking and air flow conditions, as
evaluated on the aforementioned 4-layer FR4 PCB. The
power loss curves are taken at room temperature, and
are increased with multiplicative factors with ambient
temperature. These approximate factors are: 1 up to 50°C;
1.1 for 60°C; 1.15 for 70°C; 1.2 for 80°C; 1.25 for
90°C; 1.3 for 100°C; 1.35 for 110°C and 1.4 for 120°C.
The derating curves are plotted with the output current
starting at 15A and the ambient temperature at 55°C. The
output voltages are 1.2V, 2.5V and 3.3V. These are chosen
to include the lower and higher output voltage ranges for
correlating the thermal resistance. Thermal models are
derived from several temperature measurements in a
controlled temperature chamber along with thermal mod-
eling analysis. The junction temperatures are monitored
while ambient temperature is increased with and without
air flow, and with and without a heat sink attached with
thermally conductive adhesive tape. The BGA heat sinks
evaluated in Table 5 yield very comparable performance
in laminar airflow despite being visibly different in con-
struction and form factor. The power loss increase with
ambient temperature change is factored into the derating
4611 F06
µMODULE
JUNCTION-TO-CASE (TOP)
RESISTANCE
JUNCTION-TO-BOARD RESISTANCE
JUNCTION-TO-AMBIENT RESISTANCE (JESD 51-9 DEFINED BOARD)
CASE (TOP)-TO-AMBIENT
RESISTANCE
BOARD-TO-AMBIENT
RESISTANCE
JUNCTION-TO-CASE
(BOTTOM) RESISTANCE
JUNCTION A
t
CASE (BOTTOM)-TO-BOARD
RESISTANCE
Figure 6