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LTM4653

Rev 0

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curves provided in later sections of this data sheet, along

withwell-correlatedJESD51-12-definedθvaluesprovided

in the Pin Configuration section of this data sheet.

The1V,5V,and15Vand24VpowerlosscurvesinFigures 8,

9 and 10 respectively can be used in coordination with

the load current derating curves in Figures 11 to 28 for

calculating an approximate θJA thermal resistance for the

LTM4653withvariousheatsinkingandairflowconditions.

These thermal resistances represent demonstrated

performance of the LTM4653 on DC2327A 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.TheseapproximatefactorsarelistedinTable 1.

(Compute the factor by interpolation, for intermediate

temperatures.) The derating curves are plotted with the

LTM4653’s output initially sourcing 4A and the ambient

temperature at 20°C. The output voltages are 1V, 5V,

15V and 24V. These are chosen to include the lower and

higher output voltage ranges for correlating the thermal

resistance. In all derating curves, the switching frequency

of operation follows guidance provided by Table 7.

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

ambientoperatingtemperaturespecifieshowmuchmodule

temperaturerisecanbeallowed.AsanexampleinFigure 25,

the load current is derated to 2.5A at 70°C ambient

with 200LFM airflow and no heat sink and the room

temperature (25°C) power loss for this 48VIN to 24VOUT

at 2.5AOUT condition is 3.9W. A 4.5W loss is calculated

by multiplying the 3.9W room temperature loss from the

48VIN to 24VOUT power loss curve at 2.5A (Figure 10),

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

Table 1). If the 70°C ambient temperature is subtracted

from the 120°C junction temperature, then the difference

of 50°C divided by 4.5W yields a thermal resistance, θJA,

of 11.1°C/W—in good agreement with Table 4. Tables 2,

3 and 4 provide equivalent thermal resistances for 1V, 5V

and 15V and 24V outputs with and without air flow and

heat sinking. The derived thermal resistances in Tables 2,

3 and 4 for the various conditions can be multiplied by the

calculatedpowerlossasafunctionofambienttemperature

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 1.

Table 1. 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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