LP3962 datasheet(15/34 Pages) TI

15 / 34 page

LP3962, LP3965

www.ti.com

SNVS066H – MAY 2000 – REVISED APRIL 2013

where IGND is the operating ground current of the device (specified under Electrical Characteristics).

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

LP3962 and LP3965 are available in TO-220, SFM/TO-263, and SOT-223 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, ≥60 °C/W for SFM/TO-263 package, and ≥ 140 °C/W for

SOT-223 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 PACKAGES

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 SFM/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 junction 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 SFM/TO-263 AND SOT-223 PACKAGES

The SFM/TO-263 and SOT-223 packages use the copper plane on the PCB as a heatsink. The tab of these

packages are soldered to the copper plane for heat sinking. Figure 24 shows a curve for the

θJA of SFM/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 24.

θJA vs Copper(1 Ounce) Area for SFM/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 SFM/TO-263 packag mounted to a PCB is 32°C/W.

Figure 25 shows the maximum allowable power dissipation for SFM/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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