LP3906 datasheet(37/38 Pages) NSC

37 / 38 page

Thermal Performance of the LLP

Package

The LP3906 is a monolithic device with integrated power

FETs. For that reason, it is important to pay special attention

to the thermal impedance of the LLP package and to the

PCB layout rules in order to maximize power dissipation of

the LLP package.

The LLP package is designed for enhanced thermal perfor-

mance and features an exposed die attach pad at the bottom

center of the package that creates a direct path to the PCB

for maximum power dissipation. Compared to the traditional

leaded packages where the die attach pad is embedded

inside the molding compound, the LLP reduces one layer in

the thermal path.

The thermal advantage of the LLP package is fully realized

only when the exposed die attach pad is soldered down to a

thermal land on the PCB board with thermal vias planted

underneath the thermal land. Based on thermal analysis of

the LLP package, the junction-to-ambient thermal resistance

(

θJA) can be improved by a factor of two when the die attach

pad of the LLP package is soldered directly onto the PCB

with thermal land and thermal vias, as opposed to an alter-

native with no direct soldering to a thermal land. Typical pitch

and outer diameter for thermal vias are 1.27mm and 0.33mm

respectively. Typical copper via barrel plating is 1oz, al-

though thicker copper may be used to further improve ther-

mal performance. The LP3906 die attach pad is connected

to the substrate of the IC and therefore, the thermal land and

vias on the PCB board need to be connected to ground

(GND pin).

For more information on board layout techniques, refer to

Application Note AN–1187 “Leadless Lead frame Package

(LLP).” on http://www.national.com This application note also

discusses package handling, solder stencil and the assem-

bly process.

LP3906 Datasheet(PDF) 37 Page - National Semiconductor (TI)