LP38513-ADJ Datasheet, PDF (10/12 Pages) National Semiconductor (TI) – 3A Fast-Transient Response Adjustable Low-Dropout Linear Voltage Regulator

The maximum allowable junction temperature rise (ΔT_J) de-
pends on the maximum expected ambient temperature (T_A
(MAX)^{) of the application, and the maximum allowable junction}
temperature (T_J(MAX)):
ΔT_J = T_J(MAX) − T_A(MAX)
(8)
The maximum allowable value for junction to ambient Ther-
mal Resistance, θ_JA, can be calculated using the formula:
θ_JA = ΔT_J / P_D(MAX)
(9)
LP38513-ADJ is available in the TO-263 THIN surface mount
package. For a comparison of the TO-263 THIN package to
the standard TO-263 package see Application Note AN-1797
TO-263 THIN Package. The thermal resistance depends on
amount of copper area, or heat sink, and on air flow. See Ap-
plication Note AN-1520 A Guide to Board Layout for Best
Thermal Resistance for Exposed Packages for guidelines.
Heat-Sinking the TO-263 THIN Package
The DAP of the TO-263 THIN package is soldered to the cop-
per plane for heat sinking. The TO-263 THIN package has a
θ_JA rating of 67°C/W, and a θ_JC rating of 2°C/W. The θ_JA rating
of 67°C/W includes the device DAP soldered to an area of
0.055 square inches (0.22 in x 0.25 in) of 1 ounce copper on
a two sided PCB, with no airflow. See JEDEC standard EIA/
JESD51-3 for more information.
Figure 1 shows a curve for the θ_JA of TO-263 THIN package
for different thermal via counts under the exposed DAP, using
a four layer PCB for heat sinking. The thermal vias connect
the copper area directly under the exposed DAP to the first
internal copper plane only. See JEDEC standards EIA/
JESD51-5 and EIA/JESD51-7 for more information.
Figure 2 shows the thermal performance when the Thin
TO-263 is mounted to a two layer PCB where the copper area
is predominately directly under the exposed DAP. .As shown
in the figure, increasing the copper area beyond 1 square inch
produces very little improvement.
30020736
FIGURE 2. θ_JA vs Copper Area for the TO-263 THIN
Package
30020735
FIGURE 1. θ_JA vs Thermal Via Count for the TO-263 THIN
Package on 4–Layer PCB
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