LTM8024 datasheet(18/26 Pages) AD

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LTM4709

Rev. 0

For more information www.analog.com

a thorough laboratory evaluation replicates the simulated

conditions with thermocouples within a controlled envi-

ronment chamber while operating the device at the same

power loss as simulated. An outcome of this process and

due diligence yields a set of derating curves (see Figure 7

to Figure 13).

After these laboratory tests have been performed and cor-

related to the LTM4709 model, then the θJA is provided

assuming approximately 100% of the power loss flows

from the junction through the board into the ambient with

no airflow or top-mounted heat sink.

The load current derating curves are shown in Figure 7 to

Figure 13. The maximum load current is achievable while

increasing ambient temperature if the junction tempera-

ture is less than 145°C, which is 5°C guard band from

the maximum junction temperature of 150°C. When the

ambient temperature reaches a point where the junction

temperature is 145°C, then the load current is lowered

to maintain the junction at 145°C while increasing the

ambient temperature up to 140°C. The derating curves

are plotted with three channels in parallel, total current

starting at full load 9A, and the ambient temperature at

25°C. The input-to-output differential voltages are 200mV,

250mV, 300mV, 400mV, 500mV, 600mV, 700mV. These

are chosen to include the lower and higher output voltage

ranges. Thermal models are derived from several tempera-

ture measurements in a controlled temperature chamber,

along with thermal modeling analysis. The junction tem-

peratures are monitored while the ambient temperature

is increased with and without airflow. The decreased

output current will decrease the internal module loss

as ambient temperature is increased. Table 2 provides

equivalent thermal resistances for with and without air-

flow. LTpowerCAD offers an empirical thermal model

for thermal estimation based on the EVAL-LTM4709-BZ

evaluation board as well. The printed circuit board is a

1.6mm thick four layers board with two-ounce copper for

all four layers. The PCB dimensions are 114mm × 124mm.

APPLICATIONS INFORMATION

Table 2. Derating Curve at Various VIN – VOUT

DERATING CURVE

VIN – VOUT

(V)

AIRFLOW

(LFM)

HEAT SINK

θJA

(°C/W)

Figure 7

0.2

None

Figure 8

0.25

Figure 9

0.3

Figure 10

0.4

Figure 11

0.5

Figure 12

0.6

Figure 13

0.7

Figure 7

0.2

200

None

13.5

Figure 8

0.25

Figure 9

0.3

Figure 10

0.4

Figure 11

0.5

Figure 12

0.6

Figure 13

0.7

Figure 7

0.2

400

None

Figure 8

0.25

Figure 9

0.3

Figure 10

0.4

Figure 11

0.5

Figure 12

0.6

Figure 13

0.7

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