LTM4609 datasheet(22/38 Pages) LINER

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LTM4636-1

46361fa

For more information www.linear.com/LTM4636-1

APPLICATIONS INFORMATION

curves provided in this data sheet can be used in a man-

ner that yields insight and guidance pertaining 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:

JA, 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 95mm × 76mm PCB with four layers.

JCbottom, 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.

JCtop, 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 JCbottom, this value

may be useful for comparing packages but the test

conditionsdon’tgenerallymatchtheuser’sapplication.

JB, 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 package 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

jointsandaportionoftheboard.Theboardtemperature

is measured a specified distance from the package.

A graphical representation of the aforementioned ther-

mal resistances is given in Figure 9; blue resistances are

contained within the µModule regulator, whereas green

resistances are external to the µModule package.

As a practical matter, it should be clear to the reader that

no individual or sub-group of the four thermal resistance

parameters defined by JESD51-12 or provided in the

Pin Configuration section replicates or conveys normal

operating conditions of a µModule regulator. For example,

in normal board-mounted applications, never does 100%

of the device’s total power loss (heat) thermally con-

duct exclusively through the top or exclusively through

the bottom of the µModule package—as the standard

defines for JCtop and JCbottom, respectively. In practice,

power loss is thermally dissipated in both directions away

from the package—granted, in the absence of a heat sink

and airflow, a majority of the heat flow is into the board.

Figure 9. Graphical Representation of JESD51-12 Thermal Coefficients

46361 F09

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

CASE (BOTTOM)-TO-BOARD

RESISTANCE

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