LTM8024 datasheet(33/43 Pages) AD

33 / 43 page

Data Sheet

LTM8060F

analog.com

Rev. 0

33 of 43

1. θJA is the natural convection junction-to-ambient air thermal resistance measured in 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 JESD519-defined test board,

which does not reflect an actual application or viable operating condition.

2. θJCbot is the junction-to-board thermal resistance with all the component power dissipation flowing

through the 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 environment. As a result, this

thermal resistance value may be useful for comparing packages, but the test conditions do not generally

match the user’s application.

3. θJCtop is determined with nearly all the component 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 do

not generally match the user’s application.

Given these definitions, it should now be apparent that none 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 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 the thermal resistances simultaneously.

A graphical approximation of these dominant thermal resistances is shown in Figure 68. Some thermal resistance

elements, such as heat flowing 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 LTM8060F 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 LTM8060F. The bulk of the heat flow out of the LTM8060F is

through the bottom of the package and the pads into the PCB. Consequently, a poor PCB design can cause

excessive heating, resulting in impaired performance or reliability. See the PCB Layout section for a design

suggestion.

Figure 68. Graphical Representation of Thermal Coefficients, Including the JESD5112 Terms

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

LTM8024 Datasheet(PDF) 33 Page - Analog Devices

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