LM3940IT-3.3 National Semiconductor, LM3940IT-3.3 Datasheet - Page 7

• Manufacturer Part Number: LM3940IT-3.3
• Description: IC, LDO VOLT REG, 3.3V, 1A, TO-220
• Manufacturer: National Semiconductor

Specifications of LM3940IT-3.3

• Primary Input Voltage: 5V
• Output Voltage Fixed: 3.3V
• Dropout Voltage Vdo: 110mV
• No. Of Pins: 3
• Output Current: 1A
• Operating Temperature Range: -40°C To +125°C
• Termination Type: Through Hole
• Dc: N/A
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

If a manufactured heatsink is to be selected, the value of
heatsink-to-ambient thermal resistance, θ
calculated:
θ
Where: θ
θ
When a value for θ
heatsink must be selected that has a value that is less than
or equal to this number.
θ
in the catalog, or shown in a curve that plots temperature rise
vs. power dissipation for the heatsink.
HEATSINKING TO-263 AND SOT-223 PACKAGE PARTS
Both the TO-263 (“S”) and SOT-223 (“MP”) 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 3 shows for the TO-263 the measured values of θ
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 3. θ
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
package mounted to a PCB is 32°C/W.
As a design aid, Figure 4 shows the maximum allowable
power dissipation compared to ambient temperature for the
TO-263 device (assuming θ
junction temperature is 125°C).
(H−A)
(H−A)
= θ
is specified numerically by the heatsink manufacturer
(C−H)
(JA)
(J−C)
(JA)
− θ
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
culation.
is defined as the thermal resistance between
the case and the surface of the heatsink. The
value of θ
about 2.5°C/W (depending on method of at-
tachment, insulator, etc.). If the exact value is
unknown, 2°C/W should be assumed for θ
−H)
(C−H)
vs. Copper (1 ounce) Area for the TO-263
.
(H−A)
− θ
is found using the equation shown, a
(J−C)
(C−H)
Package
(JA)
will vary from about 1.5°C/W to
is 35°C/W and the maximum
(JA)
(H−A)
1208007
(J−C)
for the TO-263
, must first be
for this cal-
(JA)
(C
7
Figure 5 and Figure 6 show the information for the SOT-223
package. Figure 6 assumes a θ
copper and 51°C/W for 2 ounce copper and a maximum junc-
tion temperature of 125°C.
FIGURE 5. θ
Please see AN1028 for power enhancement techniques to be
used with the SOT-223 package.
FIGURE 4. Maximum Power Dissipation vs. T
FIGURE 6. Maximum Power Dissipation vs. T
(JA)
vs. Copper (2 ounce) Area for the SOT-223
SOT-223 Package
TO-263 Package
Package
(JA)
of 74°C/W for 1 ounce
1208011
1208008
1208012
www.national.com
AMB
AMB
for the
for the