LP3906 datasheet(17/47 Pages) TI

17 / 47 page

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LP3906

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SNVS456M – AUGUST 2006 – REVISED MAY 2013

Figure 32. LP3906 Buck Converter

VIN vs ILOAD Operating Ranges

Thermal Performance of the WQFN 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 WQFN package and to the PCB layout rules in order to maximize

power dissipation of the WQFN package.

The WQFN package is designed for enhanced thermal performance 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 WQFN reduces one layer in the thermal path.

The thermal advantage of the WQFN 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 WQFN package, the junction-to-ambient thermal resistance (

θJA) can be improved by a

factor of two when the die attach pad of the WQFN package is soldered directly onto the PCB with thermal land

and thermal vias, as opposed to an alternative 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, although

thicker copper may be used to further improve thermal 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 (Literature Number

SNOA401) “Leadless Lead frame Package (WQFN).” This application note also discusses package handling,

solder stencil and the assembly process.

DC/DC Converters

OVERVIEW

The LP3906 supplies the various power needs of the application by means of two Linear Low Drop Regulators

LDO1 and LDO2 and two Buck converters SW1 and SW2. The table here under lists the output characteristics of

the various regulators.

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