LM2940CT-9.0 National Semiconductor, LM2940CT-9.0 Datasheet - Page 11

• Manufacturer Part Number: LM2940CT-9.0
• Description: IC,VOLT REGULATOR,FIXED,+9V,BIPOLAR,SIP,3PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2940CT-9.0

• Rohs Compliant: NO

Application Hints
an aluminum electrolytic with a solid Tantalum, with the total
capacitance split about 75/25% with the Aluminum being the
larger value.
If two capacitors are paralleled, the effective ESR is the par-
allel of the two individual values. The “flatter” ESR of the Tan-
talum will keep the effective ESR from rising as quickly at low
temperatures.
HEATSINKING
A heatsink may be required depending on the maximum
power dissipation and maximum ambient temperature of the
application. Under all possible operating conditions, the junc-
tion temperature must be within the range specified under
Absolute Maximum Ratings.
To determine if a heatsink is required, the power dissipated
by the regulator, P
The figure below shows the voltages and currents which are
present in the circuit, as well as the formula for calculating
the power dissipated in the regulator:
I
P
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T
lated by using the formula:
where: T
Using the calculated values for T
mum allowable value for the junction-to-ambient thermal re-
sistance,
IMPORTANT: If the maximum allowable value for
found to be
the TO-263 package, or 174˚C/W for the SOT-223 pack-
age, no heatsink is needed since the package alone will dis-
sipate enough heat to satisfy these requirements.
If the calculated value for
heatsink is required.
HEATSINKING TO-220 PACKAGE PARTS
The TO-220 can be attached to a typical heatsink, or se-
cured to a copper plane on a PC board. If a copper plane is
to be used, the values of
the next section for the TO-263.
IN
D
= I
= (V
L
÷ I
IN
T
A
G
− V
J
FIGURE 2. Power Dissipation Diagram
(max)
(max) is the maximum allowable junction tem-
(J−A)
OUT
53˚C/W for the TO-220 package,
T
) I
, can now be found:
R
L
+ (V
(max) = T
perature, which is 125˚C for commercial
grade parts.
is the maximum ambient temperature
which will be encountered in the applica-
tion.
D
, must be calculated.
(J−A)
IN
) I
G
= T
(J−A)
J
(max) − T
(J−A)
R
(Continued)
(max)/P
will be the same as shown in
falls below these limits, a
R
(max) and P
R
A
(max). This is calcu-
D
(max)
DS008822-37
D
, the maxi-
80˚C/W for
(J−A)
is
11
If a manufactured heatsink is to be selected, the value of
heatsink-to-ambient thermal resistance,
calculated:
Where:
When a value for
a heatsink must be selected that has a value that is less than
or equal to this number.
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
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
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
mum junction temperature is 125˚C).
(H−A)
FIGURE 3.
is specified numerically by the heatsink manufacturer
(C−H)
(J−C)
(H−A)
(J−A)
is defined as the thermal resistance from
the junction to the surface of the case. A
value of 3˚C/W can be assumed for
for this calculation.
is defined as the thermal resistance be-
tween the case and the surface of the heat-
sink. The value of
about 1.5˚C/W to about 2.5˚C/W (depend-
ing on method of attachment, insulator,
etc.). If the exact value is unknown, 2˚C/W
should be assumed for
(H−A)
=
TO-263 Package
vs Copper (1 ounce) Area for the
(J−A)
is found using the equation shown,
(J−A)
−
(C−H)
is 35˚C/W and the maxi-
−
(C−H)
(J−A)
(J−C)
(H−A)
DS008822-38
(C−H)
will vary from
for the TO-263
, must first be
.
www.national.com
(J−C)
(J−A)