LTM4680 datasheet(69/132 Pages) AD

69 / 132 page

LTM4681

Rev. 0

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APPLICATIONS INFORMATION

Many designers may opt to use laboratory equipment

and a test vehicle such as the demo board to predict the

µModule regulator’s thermal performance in their appli-

cation at various electrical and environmental operating

conditions to compliment any FEA activities. Without

FEA software, the thermal resistances reported in the Pin

Configuration section are in-and-of themselves not relevant

to providing guidance of thermal performance; instead, the

derating curves provided later in this data sheet can be

used in a manner that yields insight and guidance per-

taining to one’s application-usage, and can be adapted to

correlate thermal performance to one’s own application.

The Pin Configuration section gives four thermal coeffi-

cients explicitly defined in JESD51-12; these coefficients

are quoted or paraphrased below:

1. θJA, the thermal resistance from junction to ambient,

is the natural convection junction-to-ambient air ther-

mal resistance measured in a 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 JESD51-9 defined test board, which

does not reflect an actual application or viable operat-

ing condition.

2. θJCbottom, the thermal resistance from junction to the

bottom of the product case, is determined with all

of 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 pack-

ages but the test conditions don’t generally match the

user’s application.

3. θJCtop, the thermal resistance from junction to top of

the product case, is determined with nearly all of 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 θJCbottom, this value may

be useful for comparing packages but the test condi-

tions don’t generally match the user’s application.

4 θJB, the thermal resistance from junction to the printed

circuit board, is the junction-to-board thermal resis-

tance where almost all of the heat flows through the

bottom of the µModule regulator 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 JESD51-9.

A graphical representation of the aforementioned thermal

resistances is given in Figure 34; blue resistances are

contained within the µModule regulator, whereas green

resistances are external to the µModule package.

4681 F33

µModule DEVICE

JUNCTION-TO-CASE (TOP)

RESISTANCE

JUNCTION-TO-BOARD RESISTANCE

JUNCTION-TO-AMBIENT THERMAL RESISTANCE COMPONENTS

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION

AMBIENT

CASE (BOTTOM)-TO-BOARD

RESISTANCE

Figure 34. Graphical Representation of JESD51-12 Thermal Coefficients

LTM4680 Datasheet(PDF) 69 Page - Analog Devices

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