LTM4643 datasheet(20/30 Pages) AD

20 / 30 page

LTM4638

Rev. C

For more information www.analog.com

APPLICATIONS INFORMATION

4. θJB, the thermal resistance from junction to the printed

circuitboard,isthejunction-to-boardthermalresistance

where almost all of the heat flows through the bottom

of the µModule package and into the board, and is really

the sum of the θJCbottom and the thermal resistance of

the bottom of the part through the solder joints and

through a portion of the board. The board temperature

is measured a specified distance from the package.

A graphical representation of the aforementioned thermal

resistances is given in Figure 10; blue resistances are

contained within the μModule regulator, whereas green

resistances are external to the µModule package.

As a practical matter, it should be clear to the reader that

no individual or sub-group of the four thermal resistance

parameters defined by JESD 51-12 or provided in the

Pin Configuration section replicates or conveys normal

operating conditions of a μModule regulator. For example,

in normal board-mounted applications, never does 100%

of the device’s total power loss (heat) thermally conduct

exclusively through the top or exclusively through bot-

tom of the µModule package—as the standard defines

for θJCtop and θJCbottom, respectively. In practice, power

loss is thermally dissipated in both directions away from

the package—granted, in the absence of a heat sink and

airflow, a majority of the heat flow is into the board.

Within the LTM4638 be aware there are multiple power

devices and components dissipating power, with a con-

sequence that the thermal resistances relative to different

junctions of components or die are not exactly linear with

respect to total package power loss. To reconcile this

complicationwithoutsacrificingmodelingsimplicity—but

also,notignoringpracticalrealities—anapproachhasbeen

taken using FEA software modeling along with laboratory

testing in a controlled environment chamber to reason-

ably define and correlate the thermal resistance values

supplied in this data sheet: (1) Initially, FEA software is

used to accurately build the mechanical geometry of the

LTM4638 and the specified PCB with all of the correct

materialcoefficientsalongwithaccuratepowerlosssource

definitions; (2) this model simulates a software-defined

JEDECenvironmentconsistentwithJSED51-12topredict

power loss heat flow and temperature readings at different

interfaces that enable the calculation of the JEDEC-defined

thermal resistance values; (3) the model and FEA software

isusedtoevaluatetheLTM4638withheatsinkandairflow;

(4)havingsolvedforandanalyzedthesethermalresistance

values and simulated various operating conditions in the

softwaremodel,athoroughlaboratoryevaluationreplicates

the simulated conditions with thermocouples within a

controlledenvironmentchamberwhileoperatingthedevice

at the same power loss as that which was simulated. An

outcome of this process and due diligence yields the set

of derating curves shown in this data sheet. After these

laboratory tests have been performed and correlated to

the LTM4638 model, then the θJB and θBA are summed

together to provide a value that should closely equal the

θJA value because approximately 100% of power loss

flows from the junction through the board into ambient

with no airflow or top mounted heat sink.

The 1.0V, 1.5V, 3.3V and 5V power loss curves in Figure

11 to Figure 14 can be used in coordination with the

load current derating curves in Figure 15 to Figure 21

for calculating an approximate θJA thermal resistance

for the LTM4638 with various airflow conditions. The

power loss curves are taken at room temperature, and

are increased with a multiplicative factor according to the

ambient temperature. This approximate factor is: 1.2 for

120°C at junction temperature. Maximum load current is

achievable while increasing ambient temperature as long

as the junction temperature is less than 120°C, which is

a 5°C guard band from maximum junction temperature

of 125°C. When the ambient temperature reaches a point

where the junction temperature is 120°C, then the load

current is lowered to maintain the junction at 120°C while

increasing ambient temperature up to 120°C. The derat-

ing curves are plotted with the output current starting at

15A and the ambient temperature at 30°C. The output

voltages are 1.0V, 1.5V, 3.3V and 5V. These are chosen

to include the lower and higher output voltage ranges

for correlating the thermal resistance. Thermal models

are derived from several temperature measurements in

a controlled temperature chamber along with thermal

modeling analysis. The junction temperatures are moni-

tored while ambient temperature is increased with and

without airflow. The power loss increase with ambient

temperature change is factored into the derating curves.

The junctions are maintained at 120°C maximum while

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