Datasheet

LM2940-N, LM2940C
SNVS769H –MAY 2004–REVISED MARCH 2007 www.ti.com
It is important to note that for most capacitors, ESR is specified only at room temperature. However, the designer
must ensure that the ESR will stay inside the limits shown over the entire operating temperature range for the
design.
For aluminum electrolytic capacitors, ESR will increase by about 30X as the temperature is reduced from 25°C to
−40°C. This type of capacitor is not well-suited for low temperature operation.
Solid tantalum capacitors have a more stable ESR over temperature, but are more expensive than aluminum
electrolytics. A cost-effective approach sometimes used is to 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 junction 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
D
, must be calculated.
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
IN
= I
L
+ I
G
P
D
= (V
IN
− V
OUT
) I
L
+ (V
IN
) I
G
Figure 9. Power Dissipation Diagram
The next parameter which must be calculated is the maximum allowable temperature rise, T
R(MAX)
. This is
calculated by using the formula:
T
R(MAX)
= T
J(MAX)
− T
A(MAX)
(1)
where: T
J(MAX)
is the maximum allowable junction temperature, which is 125°C for commercial grade parts.
T
A(MAX)
is the maximum ambient temperature which will be encountered in the application.
Using the calculated values for T
R(MAX)
and P
D
, the maximum allowable value for the junction-to-ambient thermal
resistance, θ
(JA)
, can now be found:
θ
(JA)
= T
R(MAX)
/ P
D
(2)
IMPORTANT: If the maximum allowable value for θ
(JA)
is 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 package, no heatsink is needed since the
package alone will dissipate enough heat to satisfy these requirements.
If the calculated value for θ
(JA)
falls below these limits, a heatsink is required.
HEATSINKING TO-220 PACKAGE PARTS
The TO-220 can be attached to a typical heatsink, or secured to a copper plane on a PC board. If a copper plane
is to be used, the values of θ
(JA)
will be the same as shown in the next section for the TO-263.
If a manufactured heatsink is to be selected, the value of heatsink-to-ambient thermal resistance, θ
(H−A)
, must
first be calculated:
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