LTM4656 datasheet(17/28 Pages) AD

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LTM4656/LTM4656-1

Rev. 0

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APPLICATIONS INFORMATION

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configura-

tion section of the data sheet are consistent with those

parameters 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 per-

formed on a µModule 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 providing these thermal coefficients is

found in JESD51-12 (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:

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 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.

θJCbottom,thethermalresistancefromjunction-to-ambient,

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 ap-

plication or viable operating condition.

θJCtop, the thermal resistance from junction-to-top of the

productcase,isdeterminedwithnearlyallofthecomponent

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 junc-

tion 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.

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 resistance of the bottom of

the part through the solder joints and through a portion

of the board. The board temperature is measured at a

specified distance from the package, using a two-sided,

two-layer board. This board is described in JESD 51-9.

Figure 8. Graphical Representation of JESD51-12 Thermal Coefficients

4656 F08

µModule DEVICE

JUNCTION-TO-CASE (TOP)

RESISTANCE

JUNCTION-TO-BOARD RESISTANCE

JUNCTION-TO-AMBIENT RESISTANCE (JESD 51-9 DEFINED BOARD)

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION

CASE (BOTTOM)-TO-BOARD

RESISTANCE

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