LP38851 datasheet(17/29 Pages) TI

17 / 29 page

(

)

LP38851

www.ti.com

SNVS492C – JUNE 2007 – REVISED APRIL 2013

where

•

θJA is the required total thermal resistance from the junction to the ambient air, θCH is the thermal resistance

from the case to the surface of the heart-sink

•

θJC is the thermal resistance from the junction to the surface of the case

(14)

For this equation,

θJC is about 3°C/W for a SFM package. The value for θCH depends on method of attachment,

insulator, etc.

θCH varies between 1.5°C/W to 2.5°C/W. Consult the heat-sink manufacturer datasheet for details

and recommendations.

Heat-Sinking The PFM Package

The PFM package has a

θJA rating of 60°C/W, and a θJC rating of 3°C/W. These ratings are for the package only,

no additional heat-sinking, and with no airflow.

The PFM package uses the copper plane on the PCB as a heat-sink. The tab of this package is soldered to the

copper plane for heat sinking. Figure 32 shows a curve for the

θJA of PFM package for different copper area

sizes, using a typical PCB with 1 ounce copper and no solder mask over the copper area for heat-sinking.

Figure 32.

θJA vs Copper (1 Ounce) Area for the PFM package

Figure 32 shows that increasing the copper area beyond 1 square inch produces very little improvement. The

minimum value for

θJA for the PFM package mounted to a PCB is 32°C/W.

Figure 33. Maximum Power Dissipation vs Ambient Temperature for the PFM Package

Figure 33 shows the maximum allowable power dissipation for PFM packages for different ambient temperatures,

assuming

θJA is 35°C/W and the maximum junction temperature is 125°C.

Submit Documentation Feedback

Product Folder Links: LP38851

LP38851 Datasheet(PDF) 17 Page - Texas Instruments