LP38852S-ADJ Datasheet, PDF (16/29 Pages) Texas Instruments – 1.5A Fast-Response High-Accuracy Adjustable LDO Linear Regulator with Enable and Soft-Start

LP38852
SNVS482E – JANUARY 2007 – REVISED APRIL 2013
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The maximum allowable junction temperature rise (ΔT_J) depends on the maximum anticipated ambient
temperature (T_A) for the application, and the maximum allowable operating junction temperature (T_J(MAX)).
'_J = T_J(MAX) - T_A(MAX)
(12)
The maximum allowable value for junction to ambient Thermal Resistance, θ_JA, can be calculated using the
formula:
T
JA
d
'T_J
P_D
(13)
Heat-Sinking the TO-220 Package
The TO-220 package has a θ_JA rating of 60°C/W and a θ_JC rating of 3°C/W. These ratings are for the package
only, no additional heat-sinking, and with no airflow. If the needed θ_JA, as calculated above, is greater than or
equal to 60°C/W then no additional heat-sinking is required since the package can safely dissipate the heat and
not exceed the operating T_J(MAX). If the needed θ_JA is less than 60°C/W then additional heat-sinking is needed.
The thermal resistance of a TO-220 package can be reduced by attaching it to a heat sink or a copper plane on
a PC board. If a copper plane is to be used, the values of θ_JA will be same as shown in next section for DDPAK
package.
The heat-sink to be used in the application should have a heat-sink to ambient thermal resistance, θ_HA:
T ^H d T ^J - ⁽T ^C + T ^{J )}
A
A
H
C
where
• θ_JA is the required total thermal resistance from the junction to the ambient air
• θ_CH is the thermal resistance from the case to the surface of the heart-sink
• θ_JC is the thermal resistance from the junction to the surface of the case
(14)
For this equation, θ_JC is about 3°C/W for a TO-220 package. The value for θ_CH depends on method of
attachment, insulator, etc. θ_CH varies between 1.5°C/W to 2.5°C/W. Consult the heat-sink manufacturer
datasheet for details and recommendations.
Heat-Sinking the DDPAK Package
The DDPAK package has a θ_JA rating of 60°C/W, and a θ_JC rating of 3°C/W. These ratings are for the package
only, no additional heat-sinking, and with no airflow.
The DDPAK package uses the copper plane on the PCB as a heat-sink. The tab of this package is soldered to
the copper plane for heat sinking. shows a curve for the θ_JA of DDPAK package for different copper area sizes,
using a typical PCB with 1 ounce copper and no solder mask over the copper area for heat-sinking.
Figure 33. θ_JA vs Copper (1 Ounce) Area for the DDPAK package
Figure 33 shows that increasing the copper area beyond 1 square inch produces very little improvement. The
minimum value for θ_JA for the DDPAK package mounted to a PCB is 32°C/W.
16
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