Datasheet

LTM4611
19
4611fb
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applicaTions inForMaTion
Figure 7. 1.2V
OUT
Power Loss
OUTPUT CURRENT (A)
0
POWER LOSS (W)
1.5
2.0
2.5
9
15
4611 F07
1.0
0.5
0
3 6 12
3.0
3.5
4.0
5V
IN
3.3V
IN
2.5V
IN
1.8V
IN
1.5V
IN
OUTPUT CURRENT (A)
0
POWER LOSS (W)
1.5
2.0
2.5
9
15
4611 F08
1.0
0.5
0
3 6 12
3.0
3.5
4.0
5V
IN
TO 2.5V
OUT
5V
IN
TO 3.3V
OUT
3.3V
IN
TO 2.5V
OUT
AMBIENT TEMPERATURE (°C)
55
MAXIMUM LOAD CURRENT (A)
14
85
4611 F09
8
4
65 75 95
2
0
16
12
10
6
105 115 125
400LFM
200LFM
0LFM
Figure 8. 2.5V
OUT
and 3.3V
OUT
Power Loss
Figure 9. 5V
IN
to 1.2V
OUT
No Heat Sink
AMBIENT TEMPERATURE (°C)
55
MAXIMUM LOAD CURRENT (A)
14
85
4611 F10
8
4
65 75 95
2
0
16
12
10
6
105 115 125
400LFM
200LFM
0LFM
Figure 10. 5V
IN
to 1.2V
OUT
with Heat Sink
curves. The junctions are maintained at 115°C maximum
while lowering output current or power while increasing
ambient temperature. The decreased output current will
decrease the internal module loss as ambient temperature
is increased. 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 11, the load current is derated to ~12A
at ~75°C with no air or heat sink and the power loss for
the 3.3V to 1.2V at 12A output is a 2.82W loss. The 2.82W
loss is calculated with the ~2.4W room temperature loss
from the 3.3V to 1.2V power loss curve at 12A (Figure 7),
and the 1.175 multiplying factor at 75°C ambient. If the
75°C ambient temperature is subtracted from the 115°C
junction temperature, then the difference of 40°C divided
by 2.82W yields a thermal resistance, θ
JA
, of 14.2°C/W—in
good agreement with Table 2. Tables 2, 3 and 4 provide
equivalent thermal resistances for 1.2V, 2.5V and 3.3V
outputs with and without air flow and heat sinking. The
derived thermal resistances in Tables 2, 3 and 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 ambi-
ent temperature multiplicative factors.