LP2995MR datasheet(8/13 Pages) NSC

8 / 13 page

Component Selection (Continued)

the best solution when size and performance are critical,

although their cost is typically higher than any other capaci-

tor.

Capacitor recommendations for different application circuits

can be seen in the accompanying application notes with

supporting evaluation boards.

Thermal Dissipation

Since the LP2995 is a linear regulator any current flow from

TT will result in internal power dissipation generating heat.

To prevent damaging the part from exceeding the maximum

allowable junction temperature, care should be taken to

derate the part dependent on the maximum expected ambi-

ent temperature and power dissipation. The maximum allow-

able internal temperature rise (T

Rmax) can be calculated

given the maximum ambient temperature (T

Amax)ofthe

application and the maximum allowable junction temperature

Jmax).

Rmax =TJmax −TAmax

From this equation, the maximum power dissipation (P

Dmax)

of the part can be calculated:

Dmax =TRmax /

The

JA of the LP2995 will be dependent on several vari-

ables: the package used; the thickness of copper; the num-

ber of vias and the airflow. For instance, the

JA of the SO-8

is 163˚C/W with the package mounted to a standard 8x4

2-layer board with 1oz. copper, no airflow, and 0.5W dissi-

pation at room temperature. This value can be reduced to

151.2˚C/W by changing to a 3x4 board with 2 oz. copper that

is the JEDEC standard. Figure 2 shows how the

JA varies

with airflow for the two boards mentioned.

Layout is also extremely critical to maximize the output

current with the LLP package. By simply placing vias under

the DAP the

JA can be lowered significantly. Figure 3 shows

the LLP thermal data when placed on a 4-layer JEDEC

board with copper thickness of 0.5/1/1/0.5 oz. The number of

vias, with a pitch of 1.27 mm, has been increased to the

maximum of 4 where a

JA of 50.41˚C/W can be obtained.

Via wall thickness for this calculation is 0.036 mm for 1oz.

Copper.

Additional improvements in lowering the

JA can also be

achieved with a constant airflow across the package. Main-

taining the same conditions as above and utilizing the 2x2

via array, Figure 4 shows how the

JA varies with airflow.

20039321

JA vs Airflow (SO-8)

FIGURE 2.

20039322

LLP-16

JA vs # of Vias (4 Layer JEDEC Board))

FIGURE 3.

20039323

JA vs Airflow Speed (JEDEC Board with 4 Vias)

FIGURE 4.

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