Datasheet

LP3852, LP3855
www.ti.com SNVS174G –FEBRUARY 2003–REVISED APRIL 2013
LP3852 and LP3855 are available in TO-220 and TO-263 packages. The thermal resistance depends on amount
of copper area or heat sink, and on air flow. If the maximum allowable value of θ
JA
calculated above is ≥ 60 °C/W
for TO-220 package and ≥ 60 °C/W for TO-263 package no heatsink is needed since the package can dissipate
enough heat to satisfy these requirements. If the value for allowable θ
JA
falls below these limits, a heat sink is
required.
HEATSINKING TO-220 PACKAGE
The thermal resistance of a TO220 package can be reduced by attaching it to a heat sink or a copper plane on a
PC board. If a copper plane is to be used, the values of θ
JA
will be same as shown in next section for TO263
package.
The heatsink to be used in the application should have a heatsink to ambient thermal resistance,
θ
HA
≤ θ
JA
− θ
CH
− θ
JC
.
In this equation, θ
CH
is the thermal resistance from the case to the surface of the heat sink and θ
JC
is the thermal
resistance from the junction to the surface of the case. θ
JC
is about 3°C/W for a TO220 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. If the exact value
is unknown, 2°C/W can be assumed.
HEATSINKING TO-263 PACKAGE
The TO-263 package uses the copper plane on the PCB as a heatsink. The tab of these packages are soldered
to the copper plane for heat sinking. Figure 25 shows a curve for the θ
JA
of TO-263 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 25. θ
JA
vs Copper (1 Ounce) Area for TO-263 package
As shown in the figure, increasing the copper area beyond 1 square inch produces very little improvement. The
minimum value for θ
JA
for the TO-263 package mounted to a PCB is 32°C/W.
Figure 26 shows the maximum allowable power dissipation for TO-263 packages for different ambient
temperatures, assuming θ
JA
is 35°C/W and the maximum junction temperature is 125°C.
Figure 26. Maximum power dissipation vs ambient temperature for TO-263 package
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