LM3658 Operation Description

(Continued)

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

(

θ

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 1.27 mm and

0.33 mm respectively. Typical copper via barrel plating is 1

oz, although thicker copper may be used to further improve

thermal performance. The LM3658 die attach pad is con-

nected 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 1187 “Leadless Lead Frame Package

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

dling, solder stencil and the assembly process.

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