LTM4644MPY datasheet(17/36 Pages) LINER

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LTM4644/LTM4644-1

4644fe

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APPLICATIONS INFORMATION

Array Surface Mount Package Thermal Measurements”).

The motivation for providing these thermal coefficients in

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

Electronic Package Thermal Information”).

Many designers may opt to use laboratory equipment

and a test vehicle such as the demo board to predict the

µModule regulator’s thermal performance in their appli-

cation at various electrical and environmental operating

conditions to compliment any FEA activities. Without FEA

software, the thermal resistances reported in the Pin Con-

figuration 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 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 typically gives four thermal

coefficients explicitly defined in JESD 51-12; these coef-

ficients are quoted or paraphrased below:

1. 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.Thisvalueisdeterminedwiththepartmountedto

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

an actual application or viable operating condition.

2. 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 page. 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.

3. 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.

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 regulator 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 ther-

mal resistances is given in Figure 8; 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 LTM4644, 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

complication without sacrificing modeling simplicity—

but also, not ignoring practical realities—an approach

has been taken using FEA software modeling along with

laboratory testing in a controlled-environment chamber

to reasonably define and correlate the thermal resistance

valuessuppliedinthisdatasheet:(1)Initially,FEAsoftware

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