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APPLICATIONS INFORMATION
resistance for the LTM4700 with various heat sinking and
airflow conditions. These thermal resistances represent
demonstrated performance of the LTM4700 on hardware;
a 8-layer FR4 PCB measuring 99mm × 130mm × 1.6mm
using 2oz copper on all layers. The power loss curves are
taken at room temperature, and are increased with mul-
tiplicative factors of 1.35 when the junction temperature
reaches 125°C. The derating curves are plotted with the
LTM4700's paralleled outputs initially sourcing up to 100A
and the ambient temperature at 25°C. The output voltages
are 0.8V, 1.2V and 1.8V. These are chosen to include the
lower and higher output voltage ranges for correlating
the thermal resistance. Thermal models are derived from
several temperature measurements in a controlled tem-
perature chamber along with thermal modeling analysis.
The junction temperatures are monitored while ambient
temperature is increased with and without airflow.
The power loss increase with ambient temperature change
is factored into the derating curves. The junctions are
maintained at 125°C maximum while lowering output cur-
rent or power while increasing ambient temperature. The
decreased output current decreases the internal module
loss as ambient temperature is increased. The monitored
junction temperature of 125°C minus the ambient operat-
Table 10. 0.8V Output
DERATING CURVE
Figure 39, Figure 40
Figure 39, Figure 40
Figure 39, Figure 40
Table 11. 1.2V Output
DERATING CURVE
Figure 41, Figure 42
Figure 41, Figure 42
Figure 41, Figure 42
Table 12. 1.8V Output
DERATING CURVE
Figure 43, Figure 44
Figure 43, Figure 44
Figure 43, Figure 44
V
(V)
POWER LOSS CURVE
IN
5, 12
Figure 36
5, 12
Figure 36
5, 12
Figure 36
V
(V)
POWER LOSS CURVE
IN
5, 12
Figure 37
5, 12
Figure 37
5, 12
Figure 37
V
(V)
POWER LOSS CURVE
IN
5, 12
Figure 38
5, 12
Figure 38
5, 12
Figure 38
For more information
ing temperature specifies how much module temperature
rise can be allowed. As an example in Figure 40, the load
current is derated to ~80A at ~75°C ambient with no air
or heat sink and the room temperature (25°C) power loss
for this 12V
to 1.2V
IN
OUT
A 10.5W loss is calculated by multiplying the ~7.8W room
temperature loss from the 12V
curve at 80A (Figure 36), with the 1.35 multiplying factor.
If the 75°C ambient temperature is subtracted from the
125°C junction temperature, then the difference of 50°C
divided by 10.5W yields a thermal resistance, θ
4.76°C/W—in good agreement with Table 10 . Tables 10,
11 and 12 provide equivalent thermal resistances for 0.8V,
1.2V and 1.8V outputs with and without airflow and heat
sinking. The derived thermal resistances in Tables 10, 11
and 12 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 Per-
formance Characteristics section and adjusted with the
above ambient temperature multiplicative factors.
AIRFLOW (LFM)
HEAT SINK
0
None
200
None
400
None
AIRFLOW (LFM)
HEAT SINK
0
200
400
AIRFLOW (LFM)
HEAT SINK
0
200
400
www.analog.com
LTM4700
at 80A
condition is ~10.5W.
OUT
to 1.2V
power loss
IN
OUT
(°C/W)
θ
JA
4.7
3.5
3.2
(°C/W)
θ
JA
None
4.7
None
3.5
None
3.2
(°C/W)
θ
JA
None
4.7
None
3.5
None
3.2
, of
JA
Rev. B
65
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