LTM8073

20

8073fa

For more information www.linear.com/LTM8073

applicaTions inForMaTion

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.

all of the component power dissipation flowing through

the bottom of the package. In the typical µModule regula-

tor, 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 condi-

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

power dissipation flowing through the top of the pack-

age. 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

ages but the test conditions don’t generally match the

user’s application.

almost all of the heat flows through the bottom of the

µModule regulator and into the board, and is often just

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 JESD 51-9.

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 representation of these thermal resistances

is given in Figure 4. The blue resistances are contained

within the µModule regulator, and the green are outside.

The die temperature of the LTM8073 must be lower than

the maximum rating of 125°C, so care should be taken in

the layout of the circuit to ensure good heat sinking of the

LTM8073. The bulk of the heat flow out of the LTM8073 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.

Figure 4: Graphical Representation of the Thermal Resistances Between the Device Junction and Ambient

µMODULE REGULATOR

JUNCTION-TO-CASE (TOP)

RESISTANCE

JUNCTION-TO-BOARD RESISTANCE

JUNCTION-TO-AMBIENT RESISTANCE (JESD 51-9 DEFINED BOARD)

CASE (TOP)-TO-AMBIENT

RESISTANCE

BOARD-TO-AMBIENT

RESISTANCE

JUNCTION-TO-CASE

(BOTTOM) RESISTANCE

JUNCTION

AMBIENT

CASE (BOTTOM)-TO-BOARD

RESISTANCE