LTM4633 datasheet(22/36 Pages) AD

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LTM4671

Rev. B

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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

leveragetheoutcomeofthermalmodeling,simulation,and

correlationtohardwareevaluationperformedonaµModule

package mounted to a hardware test board—also defined

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

Package Thermal Measurements”). The motivation for

providingthesethermalcoefficientsinfoundinJESD51-12

(“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

applicationatvariouselectricalandenvironmentaloperat-

ing 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 the data sheet

can be used in a manner that yields insight and guidance

pertaining to one’s application-usage, and can be adapted

tocorrelatethermalperformancetoone’sownapplication.

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 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 JESD51-9 defined test board,

which does not reflect an actual application or viable

operating condition.

2. θJCbottom, the thermal resistance from junction to

ambient,isthenaturalconvectionjunction-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 JESD51-9 defined test board,

which does not reflect an actual application or viable

operating condition.

4. 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.

5. 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 JESD51-9.

Figure 8. TMON Voltage

TEMPERATURE (°C)

–50 –25

100 125

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

4671 F08

APPLICATIONS INFORMATION

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