LTM4622

20

Rev F

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APPLICATIONS INFORMATION

flows from the junction through the board into ambient

with no airflow or top mounted heat sink.

The 1V, 1.5V, 2.5V, 3.3V and 5V power loss curves in

Figures 8 to 12 can be used in coordination with the

load current derating curves in Figures 13 to 21 for cal-

LTM4622 (in two-phase single output operation) with

no heat sinking and various airflow conditions. The

power loss curves are taken at room temperature, and

are increased with multiplicative factors of 1.35 assum-

ing junction temperature at 120°C. The derating curves

are plotted with the output current starting at 5A and the

ambient temperature at 40°C. These output voltages are

chosen to include the lower and higher output voltage

ranges for correlating the thermal resistance. Thermal

models are derived from several temperature measure-

ments 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 power loss 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 tempera-

ture 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 15 the load current is derated to ~3A

at ~102°C with no air or heat sink and the power loss for

the 5V to 1.5V at 3A output is about 0.95W. The 0.95W

loss is calculated with the ~0.7W room temperature loss

from the 5V to 1.5V power loss curve at 3A, and the 1.35

multiplying factor. If the 102°C ambient temperature is

subtracted from the 120°C junction temperature, then the

thermal resistance. Table 3 specifies a 19 – 20°C/W value

which is very close. Table 2 to 6 provide equivalent ther-

mal resistances for 1V, 1.5V, 2.5V, 3.3V and 5V outputs

with and without airflow. The derived thermal resistances

in Table 2 to 6 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 multiplica-

tive 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.

Figure 22 shows a measured temperature picture of the

LTM4622 with no heatsink and no airflow, from 12V input

down to 3.3V and 5V output with 2.5A DC current on each.

Figure 22. Thermal Picture, 12V Input, 3.3V and 5V Output, 2.5A DC Each Output with No Air Flow and No Heat Sink

4622 F22