LTM4649 datasheet(20/28 Pages) LINER

20 / 28 page

LTM4648

4648f

For more information www.linear.com/LTM4648

applicaTions inForMaTion

Figure 11. No Heat Sink with

3.3VIN to 1.5VOUT

Figure 12. No Heat Sink with

5VIN to 1.5VOUT

Figure 13. No Heat Sink with

3.3VIN to 2.5VOUT

Figure 14. No Heat Sink with 5VIN to 2.5VOUT

Figure 15. No Heat Sink with 5VIN to 3.3VOUT

are maintained at 120°C maximum while lowering output

currentorpowerwithincreasingambienttemperature.The

decreasedoutputcurrentwilldecreasetheinternalmodule

loss as ambient temperature is increased. The monitored

junction temperature of 120°C minus the ambient operat-

ing temperature specifies how much module temperature

rise can be allowed. As an example in Figure 12 the load

current is derated to ~8A at ~95°C with no air or heat sink

and the power loss for the 5V to 1.5V at 8A output is about

1.68W. The 1.68W loss is calculated with the ~1.2W room

temperature loss from the 5V to 1.5V power loss curve at

8A, and the 1.4 multiplying factor at 120°C junction. If the

95°C ambient temperature is subtracted from the 120°C

junction temperature, then the difference of 25°C divided

by 1.68W equals a 15°C/W θJA thermal resistance. Table

4 specifies a 14°C/W value which is very close. Table 4,

Table 5 and Table 6 provide equivalent thermal resistances

for 1.5V, 2.5V and 3.3V outputs with and without airflow.

The derived thermal resistances in Tables 4, 5 and 6 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.Roomtemperaturepowerlosscanbederived

from the efficiency curves in the Typical Performance

AMBIENT TEMPERATURE (°C)

4648 F11

100

120

400LFM

200LFM

0LFM

AMBIENT TEMPERATURE (°C)

4648 F12

100

120

400LFM

200LFM

0LFM

AMBIENT TEMPERATURE (°C)

4648 F13

100

120

400LFM

200LFM

0LFM

AMBIENT TEMPERATURE (°C)

4648 F14

100

120

400LFM

200LFM

0LFM

AMBIENT TEMPERATURE (°C)

4648 F15

100

120

400LFM

200LFM

0LFM

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