LTM4650-1 datasheet(20/36 Pages) LINER

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LTM4636

4636f

For more information www.linear.com/LTM4636

applicaTions inForMaTion

Manydesignersmayopttouselaboratoryequipmentanda

testvehiclesuchasthedemoboardtopredicttheµModule

regulator’s thermal performance in their application at

various electrical and environmental operating conditions

to compliment any FEA activities. Without FEA software,

the thermal resistances reported in the Pin Configuration

section are, in and of themselves, not relevant to providing

guidance of thermal performance; instead, the derating

curves provided in this data sheet can be used in a man-

ner 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 gives four thermal coeffi-

cients explicitly defined in JESD51-12; 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. This environment is sometimes referred to as

“still air” although natural convection causes the air to

move. This value is determined with the part mounted

to a 95mm

× 76mm PCB with four layers.

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

bottomofthepackage.InthetypicalµModuleregulator,

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 a

portionoftheboard.Theboardtemperatureismeasured

a specified distance from the package.

A graphical representation of the aforementioned ther-

mal resistances is given in Figure 9; blue resistances are

contained within the µModule regulator, whereas green

resistances are external to the µModule package.

Figure 9. Graphical Representation of JESD51-12 Thermal Coefficients

4637 F09

µMODULE DEVICE

JUNCTION-TO-CASE (TOP)

RESISTANCE

JUNCTION-TO-BOARD RESISTANCE

JUNCTION-TO-AMBIENT THERMAL RESISTANCE COMPONENTS

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION

CASE (BOTTOM)-TO-BOARD

RESISTANCE

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