LM2940CT-9.0 National Semiconductor, LM2940CT-9.0 Datasheet - Page 14

• 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

www.national.com
Application Hints
parallel 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
parallel of the two individual values. The “flatter” ESR of the
Tantalum 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
resistance, θ
IMPORTANT: If the maximum allowable value for θ
found to be ≥ 53˚C/W for the TO-220 package, ≥ 80˚C/W for
the TO-263 package, or ≥174˚C/W for the SOT-223 pack-
age, no heatsink is needed since the package alone will
dissipate enough heat to satisfy these requirements.
If the calculated value for θ
heatsink is required.
IN
D
= I
= (V
L
÷ I
IN
T
A
G
− V
J
FIGURE 2. Power Dissipation Diagram
(max) is the maximum ambient temperature
(max) is the maximum allowable junction tem-
OUT
(JA)
T
) I
R
L
, can now be found:
+ (V
(max) = T
perature, which is 125˚C for commercial
grade parts.
which
application.
D
, must be calculated.
θ
IN
(JA)
) I
G
= T
will
J
(max) − T
R
(JA)
(max)/P
(Continued)
be
falls below these limits, a
R
(max) and P
R
encountered
A
(max). This is calcu-
D
(max)
D
, the maxi-
00882237
in
(JA)
the
is
14
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 θ
the next section for the TO-263.
If a manufactured heatsink is to be selected, the value of
heatsink-to-ambient thermal resistance, θ
calculated:
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)
FIGURE 3. θ
is specified numerically by the heatsink manufacturer
θ
(C−H)
(J−C)
θ
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 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
(H−A)
(C−H)
(JA)
(H−A)
vs. Copper (1 ounce) Area for the
.
TO-263 Package
= θ
(C−H)
is found using the equation shown,
(JA)
(JA)
(JA)
− θ
will vary from about 1.5˚C/W to
will be the same as shown in
is 35˚C/W and the maximum
(C−H)
− θ
(J−C)
(JA)
(H−A)
00882238
for the TO-263
, must first be
(J−C)
for this
(JA)