LP38843_15 Datasheet, PDF (8/17 Pages) Texas Instruments – 3A Ultra Low Dropout Linear Regulators Stable with Ceramic Output Capacitors

LP38843
SNVS290C – DECEMBER 2004 – REVISED APRIL 2013
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The maximum allowable temperature rise (T_Rmax) depends on the maximum ambient temperature (T_Amax) of the
application, and the maximum allowable junction temperature (T_Jmax):
T_Rmax = T_Jmax− T_Amax
(2)
The maximum allowable value for junction to ambient Thermal Resistance, θ_JA, can be calculated using the
formula:
θ_JA = T_Rmax / P_D
(3)
These parts are available in TO-220 and DDPAK/TO-263 packages. The thermal resistance depends on amount
of copper area or heat sink, and on air flow. If the maximum allowable value of θ_JA calculated above is ≥ 60 °C/W
for TO-220 package and ≥ 60 °C/W for DDPAK/TO-263 package no heatsink is needed since the package can
dissipate enough heat to satisfy these requirements. If the value for allowable θ_JA falls below these limits, a heat
sink is required.
HEATSINKING TO-220 PACKAGE
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/TO-263 package.
The heatsink to be used in the application should have a heatsink to ambient thermal resistance,
θ_HA≤ θ_JA − θ_CH − θ_JC.
(4)
In this equation, θ_CH is the thermal resistance from the case to the surface of the heat sink and θ_JC is the thermal
resistance from the junction to the surface of the case. 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. If the exact value
is unknown, 2°C/W can be assumed.
HEATSINKING DDPAK/TO-263 PACKAGE
The DDPAK/TO-263 package uses the copper plane on the PCB as a heatsink. The tab of this package is
soldered to the copper plane for heat sinking. The graph below shows a curve for the θ_JA of DDPAK/TO-263
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 13. θ_JA vs Copper (1 Ounce) Area for DDPAK/TO-263 package
As shown in the graph below, increasing the copper area beyond 1 square inch produces very little improvement.
The minimum value for θ_JA for the DDPAK/TO-263 package mounted to a PCB is 32°C/W.
Figure 14 shows the maximum allowable power dissipation for DDPAK/TO-263 packages for different ambient
temperatures, assuming θ_JA is 35°C/W and the maximum junction temperature is 125°C.
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