LP3963 datasheet(15/25 Pages) TI

15 / 25 page

LP3963, LP3966

www.ti.com

SNVS067H – APRIL 2000 – REVISED APRIL 2013

The maximum allowable temperature rise (TRmax) depends on the maximum ambient temperature (TAmax) of the

application, and the maximum allowable junction temperature (TJmax):

TRmax = TJmax− TAmax

The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

θJA = TRmax / PD

LP3963 and LP3966 are available in TO-220 and DDPAK/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

θJA calculated above is ≥

60 °C/W for TO-220 package and

≥ 60 °C/W for DDPAK/TO-263 package no heatsink is needed since the

package can dissipate enough heat to satisfy these requirements. If the value for allowable

θJA falls below these

limits, a heat sink is required.

HEATSINKING TO-220 PACKAGE

The thermal resistance of a TO-220 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

θJA will be same as shown in next section for

DDPAK/TO-263 package.

The heatsink to be used in the application should have a heatsink to ambient thermal resistance,

θHA≤ θJA − θCH − θJC.

In this equation,

θCH is the thermal resistance from the case to the surface of the heat sink and θJC is the thermal

resistance from the junction to the surface of the case.

θJC is about 3°C/W for a TO-220 package. The value for

θCH depends on method of attachment, insulator, etc. θCH varies between 1.5°C/W to 2.5°C/W. If the exact value

is unknown, 2°C/W can be assumed.

HEATSINKING DDPAK/TO-263 PACKAGE

The DDPAK/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 23 shows a curve for the

θJA of DDPAK/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 23.

θJA vs Copper (1 Ounce) Area for DDPAK/TO-263 package

As shown in the figure, increasing the copper area beyond 1 square inch produces very little improvement. The

minimum value for

θJA for the DDPAK/TO-263 package mounted to a PCB is 32°C/W.

Figure 24 shows the maximum allowable power dissipation for DDPAK/TO-263 packages for different ambient

temperatures, assuming

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

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