LP2995LQ datasheet(9/22 Pages) TI

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200

400

600

800

1000

AIRFLOW (Linear Feet per Minute)

SOP Board

JEDEC Board

150

160

140

170

180

100

110

120

130

LP2995

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SNVS190M – FEBRUARY 2002 – REVISED MARCH 2013

performance are critical, although their cost is typically higher than any other capacitor.

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 VTT 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 ambient temperature and power

dissipation. The maximum allowable internal temperature rise (TRmax) can be calculated given the maximum

ambient temperature (TAmax) of the application and the maximum allowable junction temperature (TJmax).

TRmax = TJmax − TAmax

From this equation, the maximum power dissipation (PDmax) of the part can be calculated:

PDmax = TRmax / θJA

The

θJA of the LP2995 will be dependent on several variables: the package used; the thickness of copper; the

number of vias and the airflow. For instance, the

θJA of the SOIC-8 is 163°C/W with the package mounted to a

standard 8x4 2-layer board with 1oz. copper, no airflow, and 0.5W dissipation 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 14 shows how the

θJA varies with airflow for the two boards mentioned.

Figure 14.

θJA vs Airflow (SOIC-8)

Layout is also extremely critical to maximize the output current with the WQFN package. By simply placing vias

under the DAP the

θJA can be lowered significantly. Figure 15 shows the WQFN 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

θJA of 50.41°C/W can be obtained. Via wall thickness for this

calculation is 0.036 mm for 1oz. Copper.

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