Datasheet

H
A
T
d
( )
J
C
C
H
J
A
TT -T +
LP38851
www.ti.com SNVS492C –JUNE 2007–REVISED APRIL 2013
where
• θ
JA
is the required total thermal resistance from the junction to the ambient air, θ
CH
is the thermal resistance
from the case to the surface of the heart-sink
• θ
JC
is the thermal resistance from the junction to the surface of the case (14)
For this equation, θ
JC
is about 3°C/W for a SFM package. The value for θ
CH
depends on method of attachment,
insulator, etc. θ
CH
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 θ
JA
rating of 60°C/W, and a θ
JC
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 θ
JA
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. θ
JA
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 θ
JA
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 θ
JA
is 35°C/W and the maximum junction temperature is 125°C.
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