LM3940IS-3.3/NOPB National Semiconductor, LM3940IS-3.3/NOPB Datasheet - Page 7

• Manufacturer Part Number: LM3940IS-3.3/NOPB
• Description: IC REGULATOR LDO 3.3V TO-263
• Manufacturer: National Semiconductor

Specifications of LM3940IS-3.3/NOPB

• Regulator Topology: Positive Fixed
• Voltage - Output: 3.3V
• Voltage - Input: Up to 7.5V
• Voltage - Dropout (typical): 0.5V @ 1A
• Number Of Regulators: 1
• Current - Output: 1A
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: TO-263-3, D²Pak (3 leads + Tab), TO-263AA
• Number Of Outputs: 1
• Polarity: Positive
• Input Voltage Max: 7.5 V
• Output Voltage: 3.3 V
• Output Type: Fixed
• Dropout Voltage (max): 0.8 V At 1 A
• Output Current: 1 A
• Line Regulation: 40 mV
• Load Regulation: 50 mV
• Maximum Operating Temperature: + 125 C
• Mounting Style: SMD/SMT
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Limit (min): -
• Lead Free Status / Rohs Status: Details
• Other names: *LM3940IS-3.3; *LM3940IS-3.3/NOPB; LM3940IS-3.3

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