lp38512tj-adj National Semiconductor Corporation, lp38512tj-adj Datasheet - Page 10

• Manufacturer Part Number: lp38512tj-adj
• Description: 1.5a Fast-transient Response Adjustable Low-dropout Linear Voltage Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
The maximum allowable junction temperature rise (ΔT
pends on the maximum expected ambient temperature (T
(MAX)
temperature (T
The maximum allowable value for junction to ambient Ther-
mal Resistance, θ
LP38512-ADJ is available in TO-263 THIN and PSOP-8 sur-
face mount packages. 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 θ
sistance depends on amount of copper area, or heat sink,
attached to the DAP, and on air flow. See Application Note
AN-1520 A Guide to Board Layout for Best Thermal Resis-
tance 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
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
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 1. θ
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.
JA
rating of 67°C/W, and a θ
) of the application, and the maximum allowable junction
JA
J(MAX)
vs Thermal Via Count for the TO-263 THIN
ΔT
Package on 4–Layer PCB
θ
JA
J
JA
, can be calculated using the formula:
= T
):
= ΔT
J(MAX)
J
/ P
JC
− T
D(MAX)
rating of 2°C/W. The θ
JA
A(MAX)
of TO-263 THIN package
30040935
JA
thermal re-
JA
rating
J
) de-
(8)
(9)
A
10
Heat-Sinking The PSOP-8 Package
The DAP of the PSOP-8 package is soldered to the copper
plane for heat sinking. The LP38512MR package has a θ
rating of 168°C/W, and a θ
of 168°C/W includes the device DAP soldered to an area of
0.008 square inches (0.09 in x 0.09 in) of 1 ounce copper on
a two sided PCB, with no airflow. See JEDEC standard EIA/
JESD51-3 for more information.
Figure 3 shows a curve 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 4 shows thermal performance for a two layer board
using thermal vias to a copper area on the bottom of the PCB.
The copper area on the top of the PCB, which is soldered to
the exposed DAP, is 0.10in x 0.20in, which is approximately
the same dimensions as the body of the PSOP-8 package.
The copper area on the bottom of the PCB is a square area
and is centered directly under the PSOP-8 package.
Package on 2–Layer PCB with Copper Area on Bottom-
FIGURE 3. θ
FIGURE 2. θ
JA
JA
vs Thermal Via Count for the PSOP-8
vs Copper Area for the TO-263 THIN
Package
JC
Side
rating of 11°C/W. The θ
30040936
30040937
JA
rating
JA