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LTM8042/LTM8042-1

80421fa

APPLICATIONS INFORMATION

Thermal Considerations

The LTM8042/LTM8042-1 output current may need to be

derated if it is required to operate in a high ambient tem-

perature or deliver a large amount of continuous power.

The amount of current derating is dependent upon the

input voltage, output power and ambient temperature. The

temperature rise curves given in the Typical Performance

Characteristics section can be used as a guide. These curves

were generated by an LTM8042/LTM8042-1 mounted to a

51cm2 4-layer FR4 printed circuit board. Boards of other

sizes and layer count can exhibit different thermal behavior,

so it is in-cumbent upon the user to verify proper operation

over the intended system’s line, load and environmental

operating conditions.

The thermal resistance numbers listed in the Pin Configura-

tion section of the data sheet are based on modeling the

μModule package mounted on a test board specified per

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

Package Thermal Measurements”). The thermal coef-

ficients provided are based on JESD 51-12 (“Guidelines

for Reporting and Using Electronic Package Thermal

Information”).

For increased accuracy and fidelity to the actual applica-

tion, many designers use finite element analysis (FEA) to

predict thermal performance. To that end, the Pin Con-

figuration section of the data sheet typically gives four

thermal coefficients:

1. θJA: thermal resistance from junction to ambient.

2. θJCBOTTOM: thermal resistance from junction to the

bottom of the product case.

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

product case.

4. θJB: thermal resistance from junction to the printed

circuit board.

While the meaning of each of these coefficients may seem to

be intuitive, JEDEC has defined each to avoid confusion and

inconsistency. These definitions are given in JESD 51-12,

and are quoted or paraphrased in the following:

1. θJA 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 is the junction-to-board thermal resistance

with all of the component power dissipation flowing

through the bottom of the package. In the typical

μModule regulator, the bulk of the heat flows out the

bottom of the package, 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 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 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 is the junction-to-board thermal resistance 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, using

a two sided, two layer board. This board is described

in JESD 51-9.

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