Data Sheet

LTM8060F

analog.com

Rev. 0

26 of 43

Power Derating

Figure 54 through Figure 56, power loss curves, can be used in coordination with the load current derating curves

(Figure 57 through Figure 65) for calculating an approximate θJA thermal resistance for the LTM8060F with airflow

conditions. The power loss curves are taken at room temperature, and are increased with a 1.35 to 1.4

multiplicative factor at 125°C. These factors come from the fact that the power loss of the regulator increases by

about 45% from 25°C to 150°C, thus a 45% spread over 125°C delta equates to ~0.35%/°C loss increase. A 125°C

maximum junction minus 25°C room temperature equates to a 100°C increase. This 100°C increase multiplied by

0.35%/°C equals a 35% power loss increase at the 125°C junction, thus the 1.35 multiplier.

The derating curves are plotted with four VOUTn at the same operating condition starting at 16A of total load current

and low ambient temperature. The derating curves with the airflow are measured at output voltages of 1.5V, 3.3V

and 5V. These are chosen to include the lower and higher output voltage ranges to correlate the thermal resistance.

Thermal models are derived from several temperature measurements in a controlled temperature chamber, along

with thermal finite element analysis (FEA) modeling.

The junction temperatures are monitored while ambient temperature is increased with and without airflow. The

power loss increases with ambient temperature change and is factored into the derating curves. The junction

temperatures are maintained at ~120°C maximum while lowering output current or power while increasing the

ambient temperature. The decreased output current will decrease the internal module loss as ambient

temperature is increased.

The derived thermal resistances in Table 8 through Table 10 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 power loss curves

and adjusted with the above ambient temperature multiplicative factors. The printed circuit board is a 1.6mm thick

6-layer board with two-ounce copper (50μm) for all the layers.

Figure 54. 1.5VOUTPower Loss Curves

Figure 55. 3.3VOUT Power Loss Curves

LOAD CURRENT (A)

0

1

2

3

4

0

1

2

3

4

12VIN

24VIN

36VIN

LOAD CURRENT (A)

0

1

2

3

4

0

1

2

3

4

12VIN

24VIN

36VIN