LTM8056 datasheet(20/26 Pages) LINER

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LTM8005

8005f

For more information www.linear.com/LTM8005

APPLICATIONS INFORMATION

JUNCTION-TO-BOARD RESISTANCE

JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

CASE(BOTTOM)-TO-BOARD

RESISTANCE

JUNCTION-TO-CASE

(TOP) RESISTANCE

CASE(TOP)-TO-BOARD

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

JUNCTION

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

µMODULE CONVERTER

AMBIENT

8005 F11

Figure11.

JCTOP is determined with nearly all of the compo-

nent power dissipation flowing through the top of the

package. As the electrical connections of the typical

µModule 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 case of JCBOTTOM, this value may be

useful for comparing packages but the test conditions

don’t generally match the user’s application.

JB 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 JCBOTTOM and the thermal resistance

of the bottom of the part through the solder joints

and through a portion of the board. The board tem-

perature is measured a specified distance from the

package, using a two sided, two layer board. This

board is described in JESD 51-9.

Given these definitions, it should now be apparent that

none 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. 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 Figure11.

The blue resistances are contained within the µModule

converter, and the green are outside.

The die temperature of the LTM8005 must be lower than

the maximum rating of 150°C, so care should be taken in

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

LTM8005. The bulk of the heat flow out of the LTM8005

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.

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