LTM8003 datasheet(18/28 Pages) AD

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LTM8051

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Power Derating

The 12VIN, 24VIN and 36VIN power loss curves can be

used in coordination with the load current derating curves

for calculating an approximate

θJA thermal resistance for

the LTM8051 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 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 6A of total load cur-

rent and low ambient temperature. The derating curves

with 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 for correlating the thermal resis-

tance. Thermal models are derived from several tempera-

ture measurements in a controlled temperature chamber

along with thermal FEA modeling.

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 while increasing ambient temperature.

The decreased output current will decrease the internal

module loss as ambient temperature is increased.

The derived thermal resistances in Tables 3 through 5

for the various conditions can be multiplied by the calcu-

lated 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 4-layer board with

two-ounce copper (70µm) for all the layers.

Table 3. 1.5V Output

DERATING CURVE

VIN (V)

POWER LOSS CURVE

AIRFLOW (LFM)

HEAT SINK

θJA (°C/W)

Graph 37-39

12, 24, 36

Graph 04

None

Graph 37-39

12, 24, 36

Graph 04

200

None

13.5

Graph 37-39

12, 24, 36

Graph 04

400

None

12.5

Table 4. 3.3V Output

DERATING CURVE

VIN (V)

POWER LOSS CURVE

AIRFLOW (LFM)

HEAT SINK

θJA (°C/W)

Graph 40-42

12, 24, 36

Graph 08

None

Graph 40-42

12, 24, 36

Graph 08

200

None

13.5

Graph 40-42

12, 24, 36

Graph 08

400

None

12.5

Table 5. 5V Output

DERATING CURVE

VIN (V)

POWER LOSS CURVE

AIRFLOW (LFM)

HEAT SINK

θJA (°C/W)

Graph 43-45

12, 24, 36

Graph 10

None

Graph 43-45

12, 24, 36

Graph 10

200

None

13.5

Graph 43-45

12, 24, 36

Graph 10

400

None

12.5

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