LTM4630 datasheet(22/38 Pages) LINER

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LTM4650A

4650afb

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APPLICATIONS INFORMATION

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section of the data sheet are consistent with those param-

eters defined by JESD51-9 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 performed on a

µModulepackagemountedtoahardwaretestboard—also

defined by JESD51-9 (“Test Boards for Area Array Surface

MountPackageThermalMeasurements”).Themotivation

for providing these thermal coefficients is 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 anticipate

the µModule regulator’s thermal performance in their ap-

plication 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 the 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 ambi-

ent, is the natural convection junction-to-ambient

air thermal resistance measured in a one cubic foot

sealed enclosure. 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 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 the junction-to-board

thermal resistance with all of the component power

dissipationflowingthroughthebottomofthepackage.

In the typical µModule, the bulk of the heat flows out

the bottom of the package, but there is always heat

flowoutintotheambientenvironment.Asaresult,this

thermal resistance value may be useful for comparing

packagesbutthetestconditionsdon’tgenerallymatch

the user’s application.

3. θJCTOP, the thermal resistance from junction to top of

the product case, is determined with nearly all of the

component power dissipation flowing through the top

of the package. As the electrical connections of the

typical µModule 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 circuit board, is the junction-to-board thermal

resistance where almost all of the heat flows through

the bottom of the µModule and into the board, and

is really the sum of the θJCbottom and the thermal re-

sistance 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, using a two sided, two layer board. This

board is described in JESD 51-9.

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.

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. For example, in normal

board-mounted applications, never does 100% of the

device’s total power loss (heat) thermally conduct exclu-

sively through the top or exclusively through bottom of the

µModule—asthestandarddefinesforθJCtopandθJCbottom,

respectively.Inpractice,powerlossisthermallydissipated

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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