LTM4622
20
Rev F
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APPLICATIONS INFORMATION
flows from the junction through the board into ambient
with no airflow or top mounted heat sink.
The 1V, 1.5V, 2.5V, 3.3V and 5V power loss curves in
Figures 8 to 12 can be used in coordination with the
load current derating curves in Figures 13 to 21 for cal-
culating an approximate θ
JA
thermal resistance for the
LTM4622 (in two-phase single output operation) with
no heat sinking and various airflow conditions. The
power loss curves are taken at room temperature, and
are increased with multiplicative factors of 1.35 assum-
ing junction temperature at 120°C. The derating curves
are plotted with the output current starting at
5A and the
ambient temperature at 40°C. These output voltages are
chosen to include the lower and higher output voltage
ranges for correlating the thermal resistance. Thermal
models are derived from several temperature measure-
ments in a controlled temperature chamber along with
thermal modeling analysis. The junction temperatures
are monitored while ambient temperature is increased
with and without airflow. The power loss increase with
ambient temperature change is factored into the derating
curves. The junctions are maintained at 120°C maximum
while lowering output current or power with increasing
ambient temperature. The decreased output current will
decrease the internal module loss as ambient tempera
-
ture is increased. The monitored junction temperature of
120°C minus the ambient operating temperature specifies
how much module temperature rise can be allowed. As an
example in Figure15 the load current is derated to ~3A
at ~102°C with no air or heat sink and the power loss for
the 5V to 1.5V at 3A output is about 0.95W. The 0.95W
loss is calculated with the ~0.7W room temperature loss
from the 5V to 1.5V power loss curve at 3A, and the 1.35
multiplying factor. If the 102°C ambient temperature is
subtracted from the 120°C junction temperature, then the
difference of 18°C divided by 0.95W equals a 19°C/W θ
JA
thermal resistance. Table3 specifies a 19 – 20°C/W value
which is very close. Table2 to 6 provide equivalent ther-
mal resistances for 1V, 1.5V
, 2.5V, 3.3V and 5V outputs
with and without airflow. The derived thermal resistances
in Table2 to 6 for the various conditions can be multiplied
by the calculated power loss as a function of ambient
temperature to derive temperature rise above ambient,
thus maximum junction temperature. Room temperature
power loss can be derived from the efficiency curves
in the Typical Performance Characteristics section and
adjusted with the above ambient temperature multiplica-
tive factors. The printed circuit board is a 1.6mm thick
four layer board with two ounce copper for the two outer
layers and one ounce copper for the two inner layers. The
PCB dimensions are 95mm × 76mm.
Figure22 shows a measured temperature picture of the
LTM4622 with no heatsink and no airflow, from 12V input
down to 3.3V and 5V output with 2.5A DC current on each.
Figure22. Thermal Picture, 12V Input, 3.3V and 5V Output, 2.5A DC Each Output with No Air Flow and No Heat Sink
4622 F22
