LTM8080

22

Rev. 0

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3. θJCtop 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 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 junction to the top of the

part. As in the case of θJCbot, this value may be useful

for comparing packages, but the test conditions don’t

generally match the user’s application.

Given these definitions, it should now be apparent that

none of these thermal coefficients reflects an actual physi-

cal operating condition of a µModule regulator. Thus, none

can be individually used to predict the product’s thermal

performance accurately. Likewise, it would be inappropri-

ate to attempt to use anyone coefficient to correlate to the

junction temperature vs load graphs given in the product’s

datasheet. The only appropriate way to use the coefficients

is when running a detailed thermal analysis, such as FEA,

which considers all thermal resistances simultaneously.

8051 F03

µModule DEVICE

θJCtop JUNCTION-TO-CASE

(TOP) RESISTANCE

θJA JUNCTION-TO-AMBIENT RESISTANCE

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

θJCbot JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION

AMBIENT

CASE (BOTTOM)-TO-BOARD

RESISTANCE

Figure 3. Graphical Representation of Thermal Coefficients, Including JESD51-12 Terms

A graphical approximation of these dominant thermal

resistances is shown in Figure 3. Some thermal resistance

elements, such as heat flow out the side of the package,

are not defined by the JEDEC standard and are not shown.

The blue resistances are contained within the µModule

regulator, and the green is outside.

The die temperature of the LTM8080 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 LTM8080.

The bulk of the heat flow out of the LTM8080 is through

the bottom of the package and the pads into the printed

circuit board. Consequently, a poorly 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.

APPLICATIONS INFORMATION