LTM8002 datasheet(20/26 Pages) LINER

20 / 26 page

LTM8065

8065fa

For more information www.linear.com/LTM8065

APPLICATIONS INFORMATION

For increased accuracy and fidelity to the actual applica-

tion, many designers use FEA (Finite Element Analysis)

to predict thermal performance. To that end, Page 2 of

the data sheet typically gives four thermal coefficients:

θJA – Thermal resistance from junction to ambient

θJCbottom – Thermal resistance from junction to the

bottom of the product case

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 the 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 simplified graphical representation of these thermal

resistances is given in Figure 6. The blue resistances are

contained within the µModule regulator, and the green

are outside.

The die temperature of the LTM8065 must be lower than

the maximum rating, so care should be taken in the layout

of the circuit to ensure good heat sinking of the LTM8065.

The bulk of the heat flow out of the LTM8065 is through

the bottom of the package and the pads into the printed

circuit board. Consequently a poor printed circuit board

design can cause excessive heating, resulting in impaired

performance or reliability. Please refer to the PCB Layout

section for printed circuit board design suggestions.

LTM8002 数据表(PDF) 20 Page - Linear Technology

LTM8002 数据表(HTML) 20 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

数据表下载