LM7812CT datasheet(12/28 Pages) TI

12 / 28 page

LM340-N, LM78xx

SNOSBT0I – FEBRUARY 2000 – REVISED MARCH 2013

www.ti.com

Figure 23. Transients

When a value for

θ(H–A) is found using the equation shown, a heatsink must be selected that has a value that is

less than or equal to this number.

θ(H–A) is specified numerically by the heatsink manufacturer in this catalog, or shown in a curve that plots

temperature rise vs power dissipation for the heatsink.

HEATSINKING DDPAK/TO-263 AND SOT-223 PACKAGE PARTS

Both the DDPAK/TO-263 (KTT) and SOT-223 (DCY) 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 plane.

shows for the DDPAK/TO-263 the measured values of

θ(J–A) for different copper area sizes using a typical PCB

with 1 ounce copper and no solder mask over the copper area used for heatsinking.

Figure 24.

θ(J–A) vs Copper (1 ounce) Area for the DDPAK/TO-263 Package

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

should also be observed that the minimum value of

θ(J–A) for the DDPAK/TO-263 package mounted to a PCB is

32°C/W.

As a design aid, Figure 25 shows the maximum allowable power dissipation compared to ambient temperature

for the DDPAK/TO-263 device (assuming

θ(J–A) is 35°C/W and the maximum junction temperature is 125°C).

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LM7812CT Datasheet(PDF) 12 Page - Texas Instruments