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LTM4618
APPLICATIONS INFORMATION
The 1.5V 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 LTM4618 with various heat
sinking and air ?ow conditions. The power loss curves
are taken at room temperature, and are increased with
multiplicative factors according to the ambient tempera-
ture. These approximate factors are: 1 for 40°C; 1.05 for
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 125°C.
The derating curves are plotted with the output current
starting at 6A and the ambient temperature at 40°C. The
output voltages are 1.5V, 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 con-
trolled temperature chamber along with thermal modeling
analysis. The junction temperatures are monitored while
ambient temperature is increased with and without air
?ow. 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 temperature is increased. The
monitored junction temperature of 120°C minus the
ambient operating temperature speci?es how much mod-
ule temperature rise can be allowed. As an example, in
Figure 11 the load current is derated to ~5A at ~85°C with
no air ?ow or heat sink and the power loss for the 12V to
1.5V at 5A output is about 1.7W. The 1.7W loss is calcu-
lated with the ~1.4W room temperature loss from the 12V
to 1.5V power loss curve at 5A, and the 1.2 multiplying
factor at 85°C ambient. If the 85°C ambient temperature
is subtracted from the 115°C junction temperature, then
the difference of 30°C divided 1.7W equals a 17°C/W θ JA
thermal resistance. Table 2 speci?es a 16°C/W value which
is very close. Table 2 and Table 3 provide equivalent thermal
resistances for 1.5V and 3.3V outputs with and without air
?ow and heat sinking. The derived thermal resistances in
Tables 2 and 3 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 ef ?ciency curves in
the Typical Performance Characteristics section and ad-
justed with the above ambient temperature multiplicative
factors. The printed circuit board is a 1.6mm thick four
layer board with two ounce copper for the two outer lay-
ers and one ounce copper for the two inner layers. The
PCB dimensions are 95mm × 76mm. The BGA heat sink
is listed in Table 3.
Safety Considerations
The LTM4618 modules do not provide isolation from V IN to
V OUT . There is no internal fuse. If required, a slow blow fuse
with a rating twice the maximum input current needs to be
provided to protect each unit from catastrophic failure.
2.0
5V IN
12V IN
3.0
12V IN
24V IN
2.5
1.5
2.0
1.0
1.5
1.0
0.5
0.5
0
0
1
2
3
4
5
6
0
0
1
2
3
4
5
6
LOAD CURRENT (A)
4618 F07
Figure 7. Power Loss at 1.5V OUT
LOAD CURRENT (A)
4618 F08
Figure 8. Power Loss at 3.3V OUT
4618fa
15
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