LTM8049 datasheet(15/20 Pages) LINER

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LTM8049

8049fa

For more information www.linear.com/LTM8049

APPLICATIONS INFORMATION

stillairalthoughnaturalconvectioncausestheairtomove.

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 thermal resistance between the junction

andbottomofthepackagewithallofthecomponentpower

dissipation flowing through the bottom of the package. In

the typical µModule converter, 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.

θJCtopisdeterminedwithnearlyallofthecomponentpower

dissipation flowing through the top of the package. As the

electricalconnectionsofthetypicalµModuleconverterare

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 converter 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 converter. 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 graphical representation of these thermal resistances

is given in Figure 3.

The blue resistances are contained within the µModule

converter, and the green are outside.

The die temperature of the LTM8049 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

LTM8049. The bulk of the heat flow out of the LTM8049

is through the bottom of the μModule converter and the

BGA 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.

8049 F03

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

AMBIENT

CASE (BOTTOM)-TO-BOARD

RESISTANCE

Figure 3. Graphical Representation of JESD51-12 Thermal Coefficients

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