LTM4643 datasheet(17/32 Pages) LINER

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LTM4643

4643fb

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Figure 7. Graphical Representation of JESD 51-12 Thermal Coefficients

4643 F07

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

AMBIENT

CASE (BOTTOM)-TO-BOARD

RESISTANCE

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 θJCbottom, this value

may be useful for comparing packages but the test

conditionsdon’tgenerallymatchtheuser’sapplication.

4. θJB,thethermalresistancefromjunctiontotheprinted

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.

A graphical representation of the aforementioned ther-

mal resistances is given in Figure 7; 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 JESD 51-12 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 conduct

exclusively through the top or exclusively through bot-

tom of the µModule package—as the standard defines

for θJCtop and θJCbottom, respectively. In practice, power

APPLICATIONS INFORMATION

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.

Within the LTM4643, be aware there are multiple power

devices and components dissipating power, with a con-

sequence that the thermal resistances relative to different

junctions of components or die are not exactly linear with

respect to total package power loss. To reconcile this

complication without sacrificing modeling simplicity—

but also, not ignoring practical realities—an approach

has been taken using FEA software modeling along with

laboratory testing in a controlled-environment chamber

to reasonably define and correlate the thermal resistance

valuessuppliedinthisdatasheet:(1)Initially,FEAsoftware

is used to accurately build the mechanical geometry of

the LTM4643 and the specified PCB with all of the cor-

rect material coefficients along with accurate power loss

source definitions; (2) this model simulates a software-

defined JEDEC environment consistent with JESD 51-12

to predict power loss heat flow and temperature readings

at different interfaces that enable the calculation of the

JEDEC-defined thermal resistance values; (3) the model

and FEA software is used to evaluate the LTM4643 with

heat sink and airflow; (4) having solved for and analyzed

these thermal resistance values and simulated various

operating conditions in the software model, a thorough

laboratory evaluation replicates the simulated conditions

with thermocouples within a controlled-environment

chamberwhileoperatingthedeviceatthesamepowerloss

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