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LTM4659

Rev. 0

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to hardware evaluation performed on a µModule pack-

age mounted to a hardware test board—also defined by

JESD51-9 (“Test Boards for Area Array Surface Mount

Package Thermal Measurements”). The motivation for

providingthesethermalcoefficientsisfoundinJESD51-12

(“Guidelines for Reporting and Using Electronic Package

Thermal Information”).

Many designers may use laboratory equipment and a test

vehicle such as the demo board to anticipate the µModule

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 on thermal performance; instead, the derating

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

ner that yields insight and guidance pertaining to one’s

application-usage,andcanbeadaptedtocorrelatethermal

performance to one’s own application.

The Pin Configuration section typically gives four thermal

coefficients explicitly defined in JESD51-12; 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 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

JESD51-9 defined test board, which does not reflect

an actual application or viable operating condition.

2. θJCbot, the thermal resistance from junction to bottom

of the product case, is determined with all of the com-

ponent power dissipation flowing through the bottom

of the package. In the typical module regulator, the bulk

of the heat flows out the bottom of the package, but

there is always heat flow out into the ambient environ-

ment. 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 the junction to the

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 θJCbot, this value may be useful

for comparing packages but the test conditions don’t

generally match the user’s application.

A graphical representation of the aforementioned thermal

resistances is shown in Figure 5; blue resistances are

contained within the µModule regulator, whereas green

resistances are external to the µModule.

APPLICATIONS INFORMATION

4659 F05

µModule DEVICE

θJCtop JUNCTION-TO-CASE

(TOP) RESISTANCE

θJA JUNCTION-TO-AMBIENT RESISTANCE

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

θJCbot JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION

AMBIENT

CASE (BOTTOM)-TO-BOARD

RESISTANCE

Figure 5. Graphical Representation of JESD51-12 Thermal Coefficients

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