LTM8003

24

8003fc

APPLICATIONS INFORMATION

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 junc-

value may be useful for comparing packages 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 really 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 JESD 51-9.

Giventhesedefinitions,itshouldnowbeapparentthatnone

of these thermal coefficients reflects an actual physical

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 6. The blue resistances are contained

within the µModule regulator, and the green are outside.

The die temperature of the LTM8003 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 LTM8003.

The bulk of the heat flow out of the LTM8003 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.

8003 F06

µMODULE DEVICE

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

Figure6. Graphical Representation of the Thermal Resistances Between the Device Junction and Ambient