Datasheet

LTM4637
18
4637fc
For more information www.linear.com/LTM4637
Figure 8. 1V
OUT
Power Loss
Figure 9. 2.5V
OUT
Power Loss
applicaTions inForMaTion
Figure 10. 5V
OUT
Power Loss
ambient temperature at ~40°C. The output voltages are
1V, 2.5V and 5V. These are chosen to include the lower,
middle and higher output voltage ranges for correlating
the thermal resistance. Thermal models are derived
from several temperature measurements in a controlled
temperature chamber along with thermal modeling
analysis. The junction temperatures are monitored while
ambient temperature is increased with and without airflow.
The p
ower lo
ss 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 13 the load current is derated to ~16A at ~80°C with
no air or heat sink and the power loss for the 12V to 1.0V at
16A output is about 4W. The 4W loss is calculated with the
~2.8W room temperature loss from the 12V to 1.0V power
loss curve at
16A, and t
he 1.4 multiplying factor at 120°C
junction. If the 80°C ambient temperature is subtracted
from the 120°C junction temperature, then the difference
of 40°C divided by 4W equals a 10°C/W θ
JA
thermal
resistance. Table 2 specifies a 9.3°C/W value which is very
close. Table 2 provides equivalent thermal resistances for
1.0V, 2.5V and 5V outputs with and without airflow and
heat sinking. The derived thermal resistances in Tables 2
thru 4 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 efficiency 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. The BGA heat sinks are listed in Table 6.
OUTPUT CURRENT (A)
0
POWER LOSS (W)
2.5
3.0
3.5
20
4637 F08
2.0
1.5
0
5
10
15
1.0
0.5
4.5
4.0
5V TO 1V P
LOSS
12V TO 1V P
LOSS
OUTPUT CURRENT (A)
0
POWER LOSS (W)
2.5
3.0
3.5
20
4637 F09
2.0
1.5
0
5
10
15
1.0
0.5
5.0
4.5
4.0
5V TO 2.5V P
LOSS
12V TO 2.5V P
LOSS
OUTPUT CURRENT (A)
0
4
5
7
15
4637 F10
3
2
5 10 20
1
0
6
POWER LOSS (W)
12V TO 5V P
LOSS
8V TO 5V P
LOSS