LP2998MASLASHNOPB datasheet(14/33 Pages) TI1

14 / 33 page

100

200

300

400

500

600

AIRFLOW (Linear Feet Per Minute)

200

400

600

800

1000

AIRFLOW (Linear Feet per Minute)

SOP Board

JEDEC Board

150

160

140

170

180

100

110

120

130

LP2998, LP2998-Q1

SNVS521K – DECEMBER 2007 – REVISED AUGUST 2014

www.ti.com

Application Information (continued)

Figure 17. RθJA vs Airflow (SOIC-8)

Additional improvements can be made by the judicious use of vias to connect the part and dissipate heat to an

internal ground plane. Using larger traces and more copper on the top side of the board can also help. With

careful layout it is possible to reduce the RθJA further than the nominal values shown in Figure 17.

Layout is also extremely critical to maximize the output current with the SO PowerPAD package. By simply

placing vias under the DAP the

θJA can be lowered significantly.

Additional improvements in lowering the RθJA can also be achieved with a constant airflow across the package.

Maintaining the same conditions as above and utilizing the 2x2 via array, Figure 18 shows how the RθJA varies

with airflow.

Figure 18. RΘJA vs Airflow Speed (Jedec Board with 4 Vias)

Optimizing the R θJA and placing the LP2998 in a section of a board exposed to lower ambient temperature

allows the part to operate with higher power dissipation. The internal power dissipation can be calculated by

summing the three main sources of loss: output current at VTT, either sinking or sourcing, and quiescent current

at AVIN and VDDQ. During the active state (when shutdown is not held low) the total internal power dissipation

can be calculated from the following equations:

PD = PAVIN + PVDDQ + PVTT

(3)

PAVIN = IAVIN * VAVIN

(4)

PVDDQ = VVDDQ * IVDDQ = VVDDQ2 x RVDDQ

(5)

To calculate the maximum power dissipation at VTT both conditions at VTT need to be examined, sinking, and

sourcing current. Although only one equation will add into the total, VTT cannot source and sink current

simultaneously.

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