LP3881 Datasheet, PDF (10/18 Pages) National Semiconductor (TI) – 0.8A Fast-Response Ultra Low Dropout Linear Regulators

LP3881
SNVS224F – MARCH 2003 – REVISED APRIL 2013
www.ti.com
The maximum allowable value for junction to ambient Thermal Resistance, θ_JA, can be calculated using the
formula:
θ_JA = T_Rmax / P_D
These parts are available in TO-220 and 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 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 TO220 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 TO263
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.
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 TO220 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 TO-263 PACKAGE
The TO-263 package uses the copper plane on the PCB as a heatsink. The tab of these packages are soldered
to the copper plane for heat sinking. The graph below shows a curve for the θ_JA of 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 20. θ_JA vs Copper (1 Ounce) Area for 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 TO-263 package mounted to a PCB is 32°C/W.
Figure 21 shows the maximum allowable power dissipation for TO-263 packages for different ambient
temperatures, assuming θ_JA is 35°C/W and the maximum junction temperature is 125°C.
10
Submit Documentation Feedback
Product Folder Links: LP3881
Copyright © 2003–2013, Texas Instruments Incorporated