Datasheet

LM2937-2.5, LM2937-3.3
www.ti.com SNVS015E –FEBRUARY 1998–REVISED APRIL 2013
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 the 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 package to
the plane.
Figure 25 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 25. 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 26 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).
Figure 26. Maximum Power Dissipation vs T
AMB
for the DDPAK/TO-263 Package
Figure 27 and Figure 28 show the information for the SOT-223 package. Figure 28 assumes a θ
(J−A)
of 74°C/W
for 1 ounce copper and 51°C/W for 2 ounce copper and a maximum junction temperature of +85°C.
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