LTC2977 datasheet(76/138 Pages) AD

76 / 138 page

LTM4664A

Rev. 0

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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 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:

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 operating

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 pack-

age, 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 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 typ-

ical µModule regulator are on the bottom of the pack-

age, 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 conditions

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 38; 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

4664A F38

µModule DEVICE

θJCTOP JUNCTION-TO-CASE

(TOP) RESISTANCE

θJB JUNCTION-TO-BOARD RESISTANCE

θJA JUNCTION-TO-AMBIENT RESISTANCE

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

θJCBOTTOM JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION

AMBIENT

CASE (BOTTOM)-TO-BOARD

RESISTANCE

Figure 38. Graphical Representation of JESD51-12 Thermal Coefficients

DUAL 25A/30A PSM APPLICATIONS INFORMATION

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