LP38853_15 Datasheet, PDF (16/28 Pages) Texas Instruments – LP38853 3A Fast-Response High-Accuracy Adjustable LDO Linear Regulator with Enable and Soft-Start

LP38853
SNVS335D – DECEMBER 2006 – REVISED APRIL 2013
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The total power dissipation is then:
P_D = P_D(PASS) + P_D(BIAS) + P_D(IN)
(11)
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 PFM Package
The PFM 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 PFM 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:
q_HA £ q_JA - (q_CH + q_JC )
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 PFM 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. Figure 33 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 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.
Figure 34 shows the maximum allowable power dissipation for DDPAK packages for different ambient
temperatures, assuming θ_JA is 35°C/W and the maximum junction temperature is 125°C.
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