LP3927 datasheet(15/16 Pages) NSC

15 / 16 page

Application Hints (Continued)

surement, junction to ambient thermal resistance (

JA) can

be improved by as much as two times if a LLP is soldered on

the board with thermal land and thermal vias than if not.

Consider the following equation:

Where P is the power dissipated, T

J is the maximum junction

temperature of the die, T

A is the ambient temperature, and

JA is the thermal resistance of the package. TJ is specified

at 150˚C.

According to the above equation, in the case where the

LP3927 is dissipating 3W of power, T

A is limited to 32.6˚C

when T

J of 125˚C and

JA of 30.8˚C/W are used in the

equation. In order to operate at a higher ambient tempera-

ture, power dissipation has to be reduced. A curve of maxi-

mum power dissipation vs ambient temperature is provided

below.

Power Dissipation vs Ambient Temperature

(

JA=30.8˚C/W)

20037915

Layout Consideration

The LP3927 has an exposed die attach pad located at the

bottom center of the LLP package. It is imperative to create

a thermal land on the PCB board when designing a PCB

layout for the LLP package. The thermal land helps to con-

duct heat away from the die, and the land should be the

same dimension as the exposed pad on the bottom of the

LLP (1:1 ratio). The land should be on both the top and the

bottom layer of the PCB board. In addition, thermal vias

should be added inside the thermal land to conduct more

heat away from the surface of the PCB to the ground plane.

Typical pitch and outer diameter for these thermal vias are

1.27 mm and 0.33 mm respectively. Typical copper via barrel

plating is 1 oz. although thicker copper may be used to

improve thermal performance. The LP3927 bottom pad is

connected to ground. Therefore, the thermal land and vias

on the PCB board need to be connected to ground.

For more information on board layout techniques, refer to

Application Note 1187 “Leadless Leadframe Package

(LLP).” The application note also discusses package han-

dling, solder stencil, and assembly process.

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