Datasheet

LM3940
www.ti.com SNVS114E –MAY 1999–REVISED MARCH 2013
I
IN
= I
L
+ I
G
P
D
= (V
IN
− V
OUT
) I
L
+ (V
IN
) I
G
Figure 20. Power Dissipation Diagram
The next parameter which must be calculated is the maximum allowable temperature rise, T
R
(max). This is
calculated by using the formula:
T
R
(max) = T
J
(max) − T
A
(max)
Where: T
J
(max)is the maximum allowable junction temperature, which is 125°C for commercial grade parts.
T
A
(max)is the maximum ambient temperature which will be encountered in the application.
Using the calculated values for T
R
(max) and P
D
, the maximum allowable value for the junction-to-ambient
thermal resistance, θ
(JA)
, can now be found:
θ
(JA)
= T
R
(max)/P
D
IMPORTANT: If the maximum allowable value for θ
(JA)
is found to be ≥ 60°C/W for the TO-220 package, ≥
80°C/W for the DDPAK/TO-263 package, or ≥174°C/W for the SOT-223 package, no heatsink is needed since
the package alone will dissipate enough heat to satisfy these requirements.
If the calculated value for θ
(JA)
falls below these limits, a heatsink is required.
HEATSINKING TO-220 PACKAGE PARTS
The TO-220 can be attached to a typical heatsink, or secured to a copper plane on a PC board. If a copper plane
is to be used, the values of θ
(JA)
will be the same as shown in the HEATSINKING TO-263 section for the
DDPAK/TO-263.
If a manufactured heatsink is to be selected, the value of heatsink-to-ambient thermal resistance, θ
(H−A)
, must
first be calculated:
θ
(H−A)
= θ
(JA)
− θ
(C−H)
− θ
(J−C)
Where: θ
(J−C)
is defined as the thermal resistance from the junction to the surface of the case. A value of 4°C/W
can be assumed for θ
(J−C)
for this calculation.
θ
(C−H)
is defined as the thermal resistance between the case and the surface of the heatsink. The value of
θ
(C−H)
will vary from about 1.5°C/W to about 2.5°C/W (depending on method of attachment, insulator, etc.).
If the exact value is unknown, 2°C/W should be assumed for θ
(C−H)
.
When a value for θ
(H−A)
is found using the equation shown above, 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 21 shows for the DDPAK/TO-263 the measured values of θ
(JA)
for different copper area sizes using a
typical PCB with 1 ounce copper and no solder mask over the copper area used for heatsinking.
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