Datasheet

LTM4616
18
4616ff
For more information www.linear.com/LTM4616
applications inForMation
Thermal Considerations and Output Current Derating
The 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 LTM4616 with various heat sinking and
airflow conditions. Both LTM4616 outputs are placed in
parallel for a total output current of 16A, and the power
loss curves are plotted for specific output voltages up to
16A. The derating curves are plotted with each output at
8A combined for a total of 16A. 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 ther
-
mal resistance. Thermal models are derived from several
temperature measurements in a controlled temperature
chamber along with thermal modeling analysis. The junc
-
tion temperatures are monitored while ambient temperature
increases with and without airflow. The junctions are
maintained at ~115°C while lowering output current or
power with increasing ambient temperature. The 115°C
value is chosen to allow for 10°C of margin relative to the
maximum temperature of 125°C. The decreased output cur
-
rent will decrease the internal module loss as ambient tem-
perature is increased. The power loss curves in Figures 7
and
8 show this
amount of power loss as a function of
load current that is specified with both channels in paral-
lel. The
monitored junction temperature of 115°C minus
the
ambient operating temperature specifies how much
module temperature rise can be allowed. As an example, in
Figure 10 the load current is derated to 10A at ~ 80°C and
the power loss for the 5V to 1.2V at 10A output is ~3.2W.
If the 80°C ambient temperature is subtracted from the
115°C maximum junction temperature, then difference of
35°C divided by 3.2W equals a 10.9°C/W. Table 4 specifies
a 10.5°C/W value which is very close. Table 4 and Table 5
provide equivalent thermal resistances for 1.2V and 3.3V
outputs, with and without airflow and heat sinking. 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 BGA heat sinks are listed below
Table 5. At load currents on each channel from 3A to 8A
(6A to 16A in parallel on the derating curves), the thermal
resistance
values in Tables 4 and 5 are fairly accurate. As
the
load currents go below the 3A level on each channel
the thermal resistance starts to increase due to the reduced
power loss on the board. The approximate thermal resis
-
tance values for these lower currents is 15°C/W.
Safety Considerations
The
LTM4616 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. The device does support thermal shutdown and
overcurrent protection.
Figure 7. 1.2V, 2.5V Power Loss Figure 8. 1.2V, 3.3V Power Loss
LOAD CURRENT (A)
0
POWER LOSS (W)
4
5
6
16
4616 F07
3
2
0
4
8
12
1
8
7
3.3V
IN
1.2V
OUT
3.3V
IN
2.5V
OUT
LOAD CURRENT (A)
0
POWER LOSS (W)
4
5
6
16
4616 F08
3
2
0
4
8
12
1
8
7
5V
IN
1.2V
OUT
5V
IN
3.3V
OUT