LTM8083 datasheet(18/24 Pages) AD

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LTM8083

Rev. 0

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APPLICATIONS INFORMATION

Table 5. 36V Output

DERATING CURVE

VIN (V)

POWER LOSS CURVE

AIR FLOW (LFM)

HEAT SINK

θJA (°C/W)

Figures 19 and 20

12, 36

Figure 13

None

Figures 19 and 20

12, 36

Figure 13

200

None

Figures 19 and 20

12, 36

Figure 13

400

None

The 3.3V, 5V, 8V, 12V, 18V, 24V and 36V power loss

curves in Figure 7 to Figure 13 can be used in coordina-

tion with the load current derating curves in Figure 14 to

Figure 20 for calculating an approximate θJA thermal resis-

tance for the LTM8083 with various airflow conditions.

The power loss curves are taken at room temperature, and

are increased with a multiplicative factor according to the

ambient temperature. This approximate factor is: 1.2 for

120°C at junction temperature. Maximum load current is

achievable while increasing ambient temperature as long

as the junction temperature is less than 120°C, which is

a 5°C guard band from maximum junction temperature

of 125°C. When the ambient temperature reaches a point

where the junction temperature is 120°C, then the load

current is lowered to maintain the junction at 120°C while

increasing ambient temperature up to 120°C. The derating

curves are plotted with the output current starting at 1.5A

and the ambient temperature at 30°C. The output voltages

are 5V, 12V, and 36V. 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 120°C maximum while lowering output cur-

rent or power with increasing ambient temperature. The

decreased output current will decrease the internal mod-

ule loss as ambient temperature is increased. The mon-

itored junction temperature of 120°C minus the ambient

operating temperature specifies how much module tem-

perature rise can be allowed. As an example, in Figure 18

the load current is derated to 0.6A at ~100°C with no air

flow or heat sink and the power loss for the 36V to 12V

at 0.6A output is about 0.84W. The 0.84W loss is calcu-

lated with the 0.7W room temperature loss from the 36V

to 12V power loss curve at 0.6A, and the 1.2 multiply-

ing factor at 120°C junction temperature. If the 100°C

ambient temperature is subtracted from the 120°C junc-

tion temperature, then the difference of 20°C divided by

0.84W equals a 23.8°C/W θJA thermal resistance. Table 4

specifies a 23°C/W value which is very close. Table 3,

Table 4, Table 5, provide equivalent thermal resistances

for 5V, 12V, and 36V outputs with and without airflow and

heat sinking. The derived thermal resistances in Table 3,

Table 4 and Table 5, 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 mul-

tiplicative factors. The printed circuit board is a 1.6mm

thick 4-layer board with two ounce copper for the two

outer layers and one ounce copper for the two inner lay-

ers. The PCB dimensions are 64mm × 64mm. A Typical

thermal image based on this PCB is shown in Figure 21.

Figure 21. Thermal Image of LTM8083 Running from

24V Input and 36V Output with 1A Load at 25°C Ambient

without Airflow or Heat Sink

8083 F21

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