Datasheet

LTM4644/LTM4644-1
19
Rev. F
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rise above ambient, thus maximum junction temperature.
Room temperature power loss can be derived from the ef-
ficiency cur
ves in the Typical Performance Characteristics
section and adjusted with the above junction temperature
multiplicative factor. 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.
The 16A represents all four channels in parallel at 4A each.
The four parallel channels have their currents reduced at
the same rate to develop an equivalent θ
JA
circuit evalu-
ation with
thermal couples or IR camera used to validate
the thermal resistance values.
Maximum Operating Ambient Temperature
Figures 30 and 31 display the Maximum Power Loss
Allowance Curves vs ambient temperature with various
heat sinking and airflow conditions. This data was derived
from the thermal impedance generated by various ther
-
mal derating
examinations with the junction temperature
measured at 120°C. This maximum power loss limitation
serves as a guideline when designing multiple output
rails with different voltages and currents by calculating
the total power loss.
For example, to determine the maximum ambient tem
-
perature when
V
OUT1
= 2.5V at 0.6A, V
OUT2
= 3.3V at 3A,
V
OUT3
= 1.8V at 1A, V
OUT4
= 1.2V at 3A, without a heat sink
and 400LFM airflow, simply add up the total power loss
for each channel read from Figure 9 to Figure 15 which in
this example equals 2.5W, then multiply by the 1.35 coef
-
ficient for
120°C junction temperature and compare the
total power loss number, 3.4W with Figure 30. Figure 30
indicates with a 3.4W total power loss, the maximum am
-
bient temperature for this particular application is around
86°C. For reference, the actual thermal derating test in the
chamber resulted in a maximum ambient temperature
of 86.3°C, very close to the calculated value. Also from
Figure 30, it is
easy to determine with a 3.4W total power
loss, the maximum ambient temperature is around 77°C
with no airflow and 81°C with 200LFM airflow.
Safety Considerations
The LTM4644 modules do not provide galvanic 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. The device does support thermal
shutdown and overcurrent protection.
APPLICATIONS INFORMATION
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