LTM4649_15 Datasheet, PDF (21/30 Pages) Linear Technology – 10A Step-Down DC/DC Module Regulator

LTM4649
Applications Information
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 temperature 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 Figure 12 the load current
is derated to ~8A at ~90°C with no air or heat sink and
the power loss for the 12V to 1.5V at 8A output is about
2.24W. The 2.24W loss is calculated with the 1.6W room
temperature loss from the 12V to 1.5V power loss curve
at 8A, and the 1.40 multiplying factor at 120°C junction. If
the 90°C ambient temperature is subtracted from the 120°C
junction temperature, then the difference of 30°C divided
by 2.24W equals a 13°C/W θ_JA thermal resistance. Table 4
specifies a 14°C/W value which is very close. Table 4 and
Table 5 provide equivalent thermal resistances for 1.5V
and 3.3V outputs with and without airflow. The derived
thermal resistances in Tables 4 and 5 for the various con-
ditions 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-
ficiency curves in the Typical Performance Characteristics
section and adjusted 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 layers and one ounce copper for the two inner layers.
The PCB dimensions are 95mm × 76mm.
Safety Considerations
The LTM4649 module does 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.
48.1°C
48.8°C
4649 F15
Figure 15. Thermal Image 12V to 1.5V at 10A
(No Heat Sink, No Air Flow. At Room Temperature Ambient)
For more information www.linear.com/LTM4649
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