LM27951SD datasheet(9/10 Pages) NSC

9 / 10 page

Application Information (Continued)

tance for the LLP-14 package. V

IN is the input voltage to the

LM27951, V

LED is the nominal LED forward voltage, and

LED is the programmed LED current.

DISSIPATION =PIN -PLED

= [Gain x V

IN x(4xILED)]−(VLED x4xILED)

J =TA +(PDISSIPATION x

JA)

The junction temperature rating takes precedence over the

ambient temperature rating. The LM27951 may be operated

outside the ambient temperature rating, so long as the junc-

tion temperature of the device does not exceed the maxi-

mum operating rating of 115˚C. The maximum ambient tem-

perature rating must be derated in applications where high

power dissipation and/or poor thermal resistance causes the

junction temperature to exceed 115˚C.

PCB Layout Considerations

The LLP is a leadframe based Chip Scale Package (CSP)

with very good thermal properties. This package has an

exposed DAP (die attach pad) at the center of the package

measuring 3.0mm x 1.6mm. The main advantage of this

exposed DAP is to offer lower thermal resistance when it is

soldered to the thermal land on the PCB. For PCB layout,

National highly recommends a 1:1 ratio between the pack-

age and the PCB thermal land. To further enhance thermal

conductivity, the PCB thermal land may include vias to a

ground plane. For more detailed instructions on mounting

LLP packages, please refer to National Semiconductor Ap-

plication Note AN-1187.

LM27951SD Datasheet(PDF) 9 Page - National Semiconductor (TI)