LTM4686BEVPBF datasheet(64/130 Pages) AD

64 / 130 page

LTM4686B

Rev. 0

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APPLICATIONS INFORMATION

The power loss curves in Figure 10 to Figure 13 can be

used in coordination with the load current derating curves

in Figure 14 to Figure 27 for calculating an approximate

θJA thermal resistance for the LTM4686B with various

heat sinking and air flow conditions. These thermal

resistances represent demonstrated performance of the

LTM4686B on DC3089A hardware; a 4-layer FR4 PCB

measuring 99mm × 133mm × 1.6mm using outer and

inner copper weights of 2oz and 1oz, 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 14. (Compute the factor by interpolation, for

intermediate temperatures.) The derating curves are

plotted with the LTM4686B’s paralleled outputs initially

sourcing up to 28A and the ambient temperature at

25°C. The output voltages are 0.6V, 1V, 1.8V, and 3.3V.

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 22, the load current is derated to 24A at 70°C

ambient with no airflow and no heat sink and the room

temperature (25°C) power loss for this 3.3VIN to 1.8VOUT

at 24AOUT condition is ~5W. A ~5.75W loss is calculated

by multiplying the ~5W room temperature loss from the

3.3VIN to 1.8VOUT power loss curve at 24A (Figure 12),

with the 1.15 multiplying factor at 70°C ambient (from

Table 14). If the 70°C ambient temperature is subtracted

from the 120°C junction temperature, then the difference

of 50°C divided by ~5.75W yields a thermal resistance,

θJA, of ~8.9°C/W—in good agreement with Table 17.

Table 15, Table 16, Table 17 and Table 18 provide equivalent

thermal resistances for 0.6V, 1V, 1.8V and 3.3V outputs

with and without air flow and heat sinking. The derived

thermal resistances in Table 15, Table 16, Table 17 and

Table 18 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 14.

Table 14. 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

LTM4686BEVPBF 数据表(PDF) 64 Page - Analog Devices

LTM4686BEVPBF 数据表(HTML) 64 Page - Analog Devices

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