LM317SSLASHNOPB datasheet(29/48 Pages) TI1

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LM117, LM317-N

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SNVS774P – MAY 2004 – REVISED OCTOBER 2015

Thermal Considerations (continued)

Figure 38. Power Dissipation Diagram

If the calculated maximum allowable thermal resistance is higher than the actual package rating, then no

additional work is needed. If the calculated maximum allowable thermal resistance is lower than the actual

package rating either the power dissipation (PD) needs to be reduced, the maximum ambient temperature TA(MAX)

needs to be reduced, the thermal resistance (

θJA) must be lowered by adding a heatsink, or some combination of

these.

If a heatsink is needed, the value can be calculated from Equation 6:

θHA ≤ (θJA – (θCH + θJC))

where

•

θCH is the thermal resistance of the contact area between the device case and the heatsink surface

•

θJC is thermal resistance from the junction of the die to surface of the package case

(6)

When a value for

θHA is found using the equation shown, a heatsink must be selected that has a value that is

less than, or equal to, this number.

The

θHA rating is specified numerically by the heatsink manufacturer in the catalog, or shown in a curve that plots

temperature rise vs power dissipation for the heatsink.

11.3.2 Heatsinking Surface Mount Packages

The TO-263 (KTT), SOT-223 (DCY) and TO-252 (NDP) packages use a copper plane on the PCB and the PCB

itself as a heatsink. To optimize the heat sinking ability of the plane and PCB, solder the tab of the package to

the plane.

11.3.2.1 Heatsinking the SOT-223 (DCY) Package

Figure 39 and Figure 40 show the information for the SOT-223 package. Figure 40 assumes a

θJA of 74°C/W for

1-oz. copper and 59.6°C/W for 2-oz. copper and a maximum junction temperature of 125°C. See AN-1028

(SNVA036) for thermal enhancement techniques to be used with SOT-223 and TO-252 packages.

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