LTM8045 datasheet(21/30 Pages) LINER

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LTM4649

4649fc

For more information www.linear.com/LTM4649

APPLICATIONS INFORMATION

while ambient temperature is increased with and without

airflow.Thepowerlossincreasewithambienttemperature

change is factored into the derating curves. The junctions

are maintained at 120°C maximum while lowering output

currentorpowerwithincreasingambienttemperature.The

decreasedoutputcurrentwilldecreasetheinternalmodule

loss as ambient temperature is increased. The monitored

junctiontemperatureof120°Cminustheambientoperating

temperature specifies how much module temperature rise

canbeallowed.AsanexampleinFigure 12theloadcurrent

is derated to ~8A at ~90°C with no air or heat sink and

the power loss for the 12V to 1.5V at 8A output is about

2.24W. The 2.24W loss is calculated with the 1.6W room

temperature loss from the 12V to 1.5V power loss curve

at 8A, and the 1.40 multiplying factor at 120°C junction. If

the90°Cambienttemperatureissubtractedfromthe120°C

junction temperature, then the difference of 30°C divided

by 2.24W equals a 13°C/W

θJA thermal resistance. Table 4

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

Table 5 provide equivalent thermal resistances for 1.5V

and 3.3V outputs with and without airflow. The derived

thermal resistances in Tables 4 and 5 for the various con-

ditions 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 ef-

ficiency curves in the Typical Performance Characteristics

section and adjusted with the above ambient temperature

multiplicative factors. The printed circuit board is a 1.6mm

thick four layer board with two ounce copper for the two

outer layers and one ounce copper for the two inner layers.

The PCB dimensions are 95mm × 76mm.

Safety Considerations

The LTM4649 module does not provide galvanic isolation

from VIN to VOUT. There is no internal fuse. If required,

a slow blow fuse with a rating twice the maximum input

current needs to be provided to protect each unit from

catastrophic failure.

Figure 15. Thermal Image 12V to 1.5V at 10A

(No Heat Sink, No Air Flow. At Room Temperature Ambient)

48.1°C

48.8°C

4649 F15

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