LP3962_15 Datasheet, PDF (15/34 Pages) Texas Instruments – 1.5A Fast Ultra Low Dropout Linear Regulators

LP3962, LP3965
www.ti.com
SNVS066H – MAY 2000 – REVISED APRIL 2013
where I_GND is the operating ground current of the device (specified under Electrical Characteristics).
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
The maximum allowable value for junction to ambient Thermal Resistance, θ_JA, can be calculated using the
formula:
θ_JA = T_Rmax / P_D
LP3962 and LP3965 are available in TO-220, SFM/TO-263, and SOT-223 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, ≥60 °C/W for SFM/TO-263 package, and ≥ 140 °C/W for
SOT-223 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 PACKAGES
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 SFM/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.
In this equation, θ_CH is the thermal resistance from the junction 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 SFM/TO-263 AND SOT-223 PACKAGES
The SFM/TO-263 and SOT-223 packages use the copper plane on the PCB as a heatsink. The tab of these
packages are soldered to the copper plane for heat sinking. Figure 24 shows a curve for the θ_JA of SFM/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 24. θ_JA vs Copper(1 Ounce) Area for SFM/TO-263 package
As shown in the figure, increasing the copper area beyond 1 square inch produces very little improvement. The
minimum value for θ_JA for the SFM/TO-263 packag mounted to a PCB is 32°C/W.
Figure 25 shows the maximum allowable power dissipation for SFM/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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