LTM4633 datasheet(17/32 Pages) LINER

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LTM4633

4633f

For more information www.linear.com/LTM4633

applicaTions inForMaTion

ensure the voltage at CNTL_PWR is always greater than

the voltage at EXTVCCatalltimesincludingduringstart-up

and shutdown. Connecting VOUT3 to EXTVCC may present

a convenient way to meet the sequencing requirement if

VOUT3 is a 5V output. Otherwise float EXTVCC if not used.

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

tools that leverage the outcome of thermal modeling,

simulation, and correlation to hardware evaluation per-

formed 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 motivation for providing these thermal coefficients is

found in JESD 51-12 (“Guidelines for Reporting and Using

Electronic Package Thermal Information”).

Many designers in lieu or to compliment any FEA activities

may opt to use laboratory equipment and a test vehicle

such as the demo board to anticipate the µModule regula-

tor’s thermal performance in their application at various

electricalandenvironmentaloperatingconditions.Without

FEA software, the thermal resistances reported in the Pin

Configurationsectionarein-and-ofthemselvesnotrelevant

toprovidingguidanceofthermalperformance;instead,the

deratingcurvesprovidedlaterinthedatasheetcanbeused

in a manner that yields insight and guidance pertaining to

one’s application usage, and can be adapted to correlate

thermal performance to one’s own application.

The Pin Configuration section provides values based on

four thermal coefficients explicitly defined in JESD 51-12;

these coefficients are quoted or paraphrased:

θJA:Thethermalresistancefromjunctiontoambient,is

the natural convection junction-to-ambient air thermal

resistance measured in a one cubic foot sealed enclo-

sure. This environment is sometimes referred to as

“still air” although natural convection causes the air to

move.Thisvalueisdeterminedwiththepartmountedto

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

an actual application or viable operating condition.

θJCbottom: The thermal resistance from the junction to

the bottom of the product case, is determined with all

of the internal power dissipation flowing through the

bottom of the package. In a 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

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 at a specified distance from the package,

using a 2-sided, 2-layer board. This board is described

in JESD 51-9.

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