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

• Manufacturer Part Number: LM3940IS-3.3
• Description: V REG LDO +3.3V, 3940, SOT-263-3
• Manufacturer: National Semiconductor

Specifications of LM3940IS-3.3

• Primary Input Voltage: 5V
• Output Voltage: 3.3V
• Dropout Voltage Vdo: 110mV
• No. Of Pins: 3
• Output Current: 1A
• Voltage Regulator Case Style: SOT-263
• Operating Temperature Range: -40°C To +125°C
• Svhc: No
• Output Voltage Fixed: 3.3V
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application Hints
If a manufactured heatsink is to be selected, the value of
heatsink-to-ambient thermal resistance, θ
calculated:
θ
Where: θ
When a value for θ
a 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.
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)
FIGURE 3. θ
= θ
is specified numerically by the heatsink manufacturer
θ
(C−H)
(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
calculation.
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
(C−H)
(JA)
(C−H)
(H−A)
vs. Copper (1 ounce) Area for the
.
TO-263 Package
− θ
(C−H)
is found using the equation shown,
(J−C)
(JA)
will vary from about 1.5˚C/W to
is 35˚C/W and the maximum
(Continued)
(JA)
(H−A)
01208007
for the TO-263
, must first be
(J−C)
for this
(JA)
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
junction temperature of 125˚C.
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
FIGURE 5. θ
(JA)
the SOT-223 Package
the TO-263 Package
vs. Copper (2 ounce) Area for the
SOT-223 Package
(JA)
of 74˚C/W for 1 ounce
01208011
01208008
01208012
www.national.com
AMB
AMB
for
for