LP38853_15 datasheet(16/28 Pages) TI

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LP38853

SNVS335D – DECEMBER 2006 – REVISED APRIL 2013

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The total power dissipation is then:

PD = PD(PASS) + PD(BIAS) + PD(IN)

(11)

The maximum allowable junction temperature rise (

ΔTJ) depends on the maximum anticipated ambient

temperature (TA) for the application, and the maximum allowable operating junction temperature (TJ(MAX)) .

(12)

The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

(13)

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. If the needed

θJA, as calculated above, is greater than or equal to

60°C/W then no additional heat-sinking is required since the package can safely dissipate the heat and not

exceed the operating TJ(MAX). If the needed θJA is less than 60°C/W then additional heat-sinking is needed.

The thermal resistance of a PFM 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

package.

The heat-sink to be used in the application should have a heat-sink to ambient thermal resistance,

θHA:

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 PFM 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 DDPAK Package

The DDPAK 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 DDPAK 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 33 shows a curve for the

θJA of DDPAK 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 33 shows that increasing the copper area beyond 1 square inch produces very little improvement. The

minimum value for

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

Figure 34 shows the maximum allowable power dissipation for DDPAK packages for different ambient

temperatures, assuming

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

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