LTM4643 datasheet(22/28 Pages) AD

22 / 28 page

LTM8051

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

a 74cm2 4-layer FR4 printed circuit board. Boards of

other sizes and layer count can exhibit different thermal

behavior, so it is incumbent upon the user to verify proper

operation over the intended system’s line, load and envi-

ronmental operating conditions.

For increased accuracy and fidelity to the actual applica-

tion, many designers use FEA (Finite Element Analysis) or

CFD (Computational Fluid Dynamics) to predict thermal

performance. To that end, the Pin Configuration typically

gives three dominant thermal coefficients:

1. θJA – Thermal resistance from junction to ambient

2. θJCbot – Thermal resistance from junction to the bot-

tom of the product case

3. θJCtop – Thermal resistance from junction to top of

the product case

While the meaning of each of these coefficients may seem

to be intuitive, JEDEC has defined each to avoid confusion

and inconsistency. These definitions are given in JESD

51-12, and are quoted or paraphrased below:

1. θJA is the natural convection junction-to-ambient

air thermal 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 JESD 51-9 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 of the component power dissipation flow-

ing 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 compar-

ing packages but the test conditions don’t generally

match the user’s application.

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

of them can be individually used to accurately predict the

thermal performance of the product. Likewise, it would

be inappropriate to attempt to use any one coefficient to

correlate to the junction temperature vs load graphs given

in the product’s data sheet. The only appropriate way to

use the coefficients is when running a detailed thermal

analysis, such as FEA, which considers all of the thermal

resistances simultaneously.

A graphical approximation of these dominant thermal

resistances is given in Figure 4. Some thermal resis-

tance 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 are outside.

The die temperature of the LTM8051 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 LTM8051.

The bulk of the heat flow out of the LTM8051 is through

the bottom of the package and the pads into the printed

circuit board. Consequently a poor 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.

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