LTM8049 datasheet(22/34 Pages) LINER

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LTM4651

4651f

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conditions with thermocouples within a controlled envi-

ronment chamber while operating the device at the same

power loss as that which was simulated. The outcome of

this process and due diligence yields the set of derating

curves provided in later sections of this data sheet, along

withwell-correlatedJESD51-12-definedθvaluesprovided

in the Pin Configuration section of this data sheet.

The –5V, –15V and –24V power loss curves in Figures 10,

11 and 12 respectively can be used in coordination with

the load current derating curves in Figures 13 to 30 for

calculating an approximate θJA thermal resistance for the

LTM4651withvariousheatsinkingandairflowconditions.

These thermal resistances represent demonstrated

performance of the LTM4651 on DC2328A hardware; a

4-layerFR4PCBmeasuring99mm×133mm×1.6mmusing

outerandinnercopperweightsof2ozand1oz,respectively.

The power loss curves are taken at room temperature,

and are increased with multiplicative factors with ambient

temperature.TheseapproximatefactorsarelistedinTable2.

(Compute the factor by interpolation, for intermediate

temperatures.) The derating curves are plotted with the

LTM4651’s output initially sourcing its maximum output

capability(seeEq.5)andtheambienttemperatureat30°C.

The output voltages are –5V, –15V and –24V. These are

chosen to include the lower and higher output voltage

ranges for correlating the thermal resistance. In all derat-

ing curves, the switching frequency of operation follows

guidance provided by Table 1. Thermal models are derived

from several temperature measurements in a controlled

temperaturechamberalongwiththermalmodelinganalysis.

The junction temperatures are monitored while ambient

temperatureisincreasedwithandwithoutairflow,andwith

andwithoutaheatsinkattachedwiththermallyconductive

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

loweringoutputcurrentorpowerwhileincreasingambient

temperature. The decreased output current decreases the

internal module loss as ambient temperature is increased.

The monitored junction temperature of 120°C minus the

ambientoperatingtemperaturespecifieshowmuchmodule

temperaturerisecanbeallowed.AsanexampleinFigure26,

the load current is derated to 1A at 60°C ambient with

200LFMairflowandnoheatsinkandtheroomtemperature

(25°C) power loss for this 12VIN to –24VOUT at 1A out

conditionis3.55W.A3.9Wlossiscalculatedbymultiplying

the 3.55W room temperature loss from the 12VIN to

–24VOUT power loss curve at 1A (Figure 12), 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 3.9W yields a thermal resistance, θJA, of 15.4°C/W—in

good agreement with Table 4. Tables 3, 4 and 5 provide

equivalent thermal resistances for –5V, –15V and –24V

outputs with and without air flow and heat sinking. The

derived thermal resistances in Tables 3, 4 and 5 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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