LTM8048 datasheet(17/22 Pages) LINER

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LTM8045

8045fa

For more information www.linear.com/8045

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θJCtop is determined with nearly all of the component

power dissipation flowing through the top of the pack-

age.AstheelectricalconnectionsofthetypicalµModule

converter 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

caseof

θJCbottom,thisvaluemaybeusefulforcomparing

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

throughaportionoftheboard.Theboardtemperatureis

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

APPLICATIONS INFORMATION

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 4.

The blue resistances are contained within the µModule

converter, and the green are outside.

The die temperature of the LTM8045 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

LTM8045. The bulk of the heat flow out of the LTM8045

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.

8045 F04

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 4.

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