LTM4616
APPLICATIONS INFORMATION
Thermal Considerations and Output Current Derating
moduletemperaturerisecanbeallowed.Asanexample,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°Cdividedby3.2Wequalsa10.9°C/W. Table4specifies
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
ouncecopperforthetwoinnerlayers.ThePCBdimensions
are 95mm × 76mm. The BGA heat sinks are listed below
Table 5. At load currents on each channel from 3A to 8A
(6A to16A 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
thethermalresistancestartstoincreaseduetothereduced
power loss on the board. The approximate thermal resis-
tance values for these lower currents is 15°C/W.
The power loss curves in Figures 7 and 8 can be used
in coordination with the load current derating curves in
Figures 9 to16 for calculating an approximate θ thermal
JA
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, 2.5Vand3.3V. Thesearechosentoincludethelower
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-
tiontemperaturesaremonitoredwhileambienttemperature
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
maximumtemperatureof125°C.Thedecreasedoutputcur-
rentwilldecreasetheinternalmodulelossasambienttem-
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
Safety Considerations
The LTM4616 modules do not provide isolation from V
IN
to V . There is no internal fuse. If required, a slow blow
OUT
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.
8
7
6
5
4
3
2
8
7
6
5
4
3
2
1
1
5V 1.2V
3.3V 1.2V
IN
OUT
OUT
IN
OUT
OUT
5V 3.3V
IN
3.3V 2.5V
IN
0
0
8
0
4
12
16
8
0
4
12
16
LOAD CURRENT (A)
LOAD CURRENT (A)
4616 F08
4616 F07
Figure 7. 1.2V, 2.5V Power Loss
Figure 8. 1.2V, 3.3V Power Loss
4616fe
18
For more information www.linear.com/LTM4616
LINEAR_DIMENSIONS [ Linear Dimensions ]
