LTM4627_15 datasheet(17/32 Pages) LINER

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LTM4627

4627fc

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APPLICATIONS INFORMATION

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configura-

tion section of the data sheet are consistent with those

parameters defined by JESD 51-12 and are intended for

use with finite element analysis (FEA) software modeling

toolsthatleveragetheoutcomeofthermalmodeling,simu-

lation, and correlation to hardware evaluation performed

on a µModule package mounted to a hardware test board

defined by JESD 51-9 (“Test Boards for Area Array Surface

Mount Package Thermal Measurements”). The motiva-

tion for providing these thermal coefficients is found in

JESD51-12(“GuidelinesforReportingandUsingElectronic

Package Thermal Information”).

Many designers, in lieu of or to compliment any FEA ac-

tivities, may opt to use laboratory equipment and a test

vehicle such as the demo board to anticipate the µModule

regulator’s thermal performance in their application at

variouselectricalandenvironmentaloperatingconditions.

Without FEA software, the thermal resistances reported in

thePinConfigurationsectionarein-and-ofthemselvesnot

relevant to providing guidance of thermal performance;

instead,thederatingcurvesprovidedlaterinthisdatasheet

can be used in a manner that yields insight and guidance

pertaining to one’s application-usage, and can be adapted

tocorrelatethermalperformancetoone’sownapplication.

The Pin Configuration section gives four thermal coeffi-

cients explicitly defined in JESD 51-12; these coefficients

are quoted or paraphrased below:

θJA, the thermal resistance from junction to ambient, is

the natural convection junction-to-ambient air thermal

resistance measured in a one cubic foot sealed enclo-

sure.Thisenvironmentissometimesreferredtoas“still

air” although natural convection causes the air to move.

This value is determined with the part mounted to a

JESD 51-9 defined test board, which does not reflect

an actual application or viable operating condition.

θJCbottom, the thermal resistance from junction to the

bottom of the product case, is determined with all of

the component power dissipation flowing through the

bottom of the package. In the typical µModule regulator,

the bulk of the heat flows out the bottom of the pack-

age, but there is always heat flow out into the ambient

environment. As a result, this thermal resistance value

may be useful for comparing packages but the test

conditions don’t generally match the user’s application.

θJCtop, the thermal resistance from junction to top of

the product case, is determined with nearly all of the

componentpowerdissipationflowingthroughthetopof

the package. As the electrical connections of the typical

µModule regulator are on the bottom of the package, it

is rare for an application to operate such that most of

the heat flows from the junction to the top of the part.

As in the case of

θJCbottom, this value may be useful

for comparing packages but the test conditions don’t

generally match the user’s application.

θJB, the thermal resistance from junction to the printed

circuit board, is the junction-to-board thermal resis-

tance 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 at a specified distance from

the package, using a two sided, two layer board. This

board is described in JESD 51-9.

A graphical representation of the aforementioned ther-

mal resistances is given in Figure 6; 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.

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