LTM4613 datasheet(20/28 Pages) LINER

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LTM8001

8001f

For more information www.linear.com/8001

θJCtop: Thermal resistance from junction to top of the

product case

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 below:

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.

θJCbottom is the junction-to-board thermal resistance with

allofthecomponentpowerdissipationflowingthroughthe

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 envi-

ronment. 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.

θJCtopisdeterminedwithnearlyallofthecomponentpower

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 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 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.

Giventhesedefinitions,itshouldnowbeapparentthatnone

of these thermal coefficients reflects an actual physical

operating condition of a µModule regulator. Thus, none

of them can be individually used to accurately predict the

thermal performance of the product. Likewise, it would

be inappropriate to attempt to use any one coefficient to

correlate to the junction temperature vs load graphs given

in this product’s data sheet. The only appropriate way to

use the coefficients is when running a detailed thermal

analysis, such as FEA, which considers all of the thermal

resistances simultaneously.

A graphical representation of these thermal resistances

is Figure 8. The blue resistances are contained within the

µModule regulator, and the green are outside.

The die temperature of the LTM8001 must be lower than

the maximum rating of 125°C, so care should be taken in

the layout of the circuit to ensure good heat sinking of the

LTM8001. The bulk of the heat flow out of the LTM8001

is through the bottom of the module and the BGA pads

into the printed circuit board. Consequently a poor printed

circuit board design can cause excessive heating, result-

ing in impaired performance or reliability. Please refer to

the PCB Layout section for printed circuit board design

suggestions.

APPLICATIONS INFORMATION

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