LM340-N, LM78xx
SNOSBT0I –FEBRUARY 2000–REVISED MARCH 2013
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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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