LM2940 datasheet(14/19 Pages) NSC

14 / 19 page

Application Hints (Continued)

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

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:

The next parameter which must be calculated is the maxi-

mum allowable temperature rise, T

R (max). This is calcu-

lated by using the formula:

R (max) = TJ(max) − TA (max)

where: T

J (max) is the maximum allowable junction tem-

perature, which is 125˚C for commercial

grade parts.

A (max)

is the maximum ambient temperature

which

will

encountered

the

application.

Using the calculated values for T

R(max) and PD, the maxi-

mum allowable value for the junction-to-ambient thermal

resistance,

(JA), can now be found:

(JA) =TR (max)/PD

IMPORTANT: If the maximum allowable value for

(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 pack-

age, no heatsink is needed since the package alone will

dissipate enough heat to satisfy these requirements.

If the calculated value for

(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 se-

cured to a copper plane on a PC board. If a copper plane is

to be used, the values of

(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,

(H−A), must first be

calculated:

(H−A) =

(JA) −

(C−H) −

(J−C)

Where:

(J−C) 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

(J−C) for this

calculation.

(C−H)

is defined as the thermal resistance between

the case and the surface of the heatsink. The

value of

(C−H) will vary from about 1.5˚C/W to

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

(C−H).

When a value for

(H−A) is found using the equation shown,

a heatsink must be selected that has a value that is less than

or equal to this number.

(H−A) is specified numerically by the heatsink manufacturer

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

(JA)

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

(JA) for the TO-263

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

(JA) is 35˚C/W and the maximum

junction temperature is 125˚C).

00882237

IIN =IL ÷IG

PD =(VIN −VOUT)IL +(VIN)IG

FIGURE 2. Power Dissipation Diagram

00882238

FIGURE 3.

(JA) vs. Copper (1 ounce) Area for the

TO-263 Package

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