LTM8045 datasheet(37/54 Pages) AD

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LTM4655

Rev. 0

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APPLICATIONS INFORMATION

respectively. The power loss curves are taken at room

temperature, and are increased with multiplicative factors

with ambient temperature. These approximate factors are

listed in Table 1. (Compute the factor by interpolation,

for intermediate temperatures.) The derating curves

are plotted with the LTM4655’s outputs paralleled and

inteleaved, sourcing its maximum output capability, in

an environment with temperature-controlled ambient.

The output voltages are 1VOUT, 5VOUT, 15VOUT, –5VOUT,

–15VOUT and –24VOUT. 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

temperature chamber along with thermal modeling

analysis. The junction temperatures are monitored while

ambient temperature is increased with and without air

flow, and with and without a heat sink attached with

thermally conductive adhesive tape. 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 decreases the internal module loss as

ambient temperature 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 30, the load

current is derated to 3.05A per channel (6.1A, combined)

at 60°C ambient with no airflow and no heat sink and the

room temperature (25°C) per channel power loss for this

24VIN to –5VOUT at 3.05A out condition is 2.45W; 4.9W,

combined. A 5.39W loss is calculated by multiplying the

4.9W room temperature loss from the 24VIN to –5VOUT

power loss curve at 4.9W (Figure 25), with the 1.1

multiplying factor at 60°C ambient (from Table 2). If the

60°C ambient temperature is subtracted from the 120°C

junction temperature, then the difference of 60°C divided

by 5.39W yields a thermal resistance, θJA, of 11.1°C/W—

in good agreement with Table 6. Table 3 to Table 5 provide

equivalent thermal resistances for 1V, 5V, and 15V outputs

with and without airflow and heatsinking. Table 6 to

Table 8 provide equivalent thermal resistances for –5V,

–15V and –24V outputs with and without airflow and

heatsinking. The derived thermal resistances in Table 3

to Table 8 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 ambient temperature multiplicative factors

from Table 2.

Table 2. Power Loss Multiplicative Factors vs Ambient

Temperature

AMBIENT TEMPERATURE

POWER LOSS MULTIPLICATIVE

FACTOR

Up to 40°C

1.00

50°C

1.05

60°C

1.10

70°C

1.15

80°C

1.20

90°C

1.25

100°C

1.30

110°C

1.35

120°C

1.40

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