LP3881 Datasheet, PDF (9/12 Pages) National Semiconductor (TI) – 0.8A Fast-Response Ultra Low Dropout Linear Regulators

Application Hints (Continued)
POWER DISSIPATION/HEATSINKING
A heatsink may be required depending on the maximum
power dissipation and maximum ambient temperature of the
application. Under all possible conditions, the junction tem-
perature must be within the range specified under operating
conditions. The total power dissipation of the device is given
by:
P_D = (V_IN−V_OUT)I_OUT+ (V_IN)I_GND
where I_GND is the operating ground current of the device.
The maximum allowable temperature rise (T_Rmax) depends
on the maximum ambient temperature (T_Amax) of the appli-
cation, and the maximum allowable junction temperature
(T_Jmax):
T_Rmax = T_Jmax− T_Amax
The maximum allowable value for junction to ambient Ther-
mal 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 resis-
tance 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 de-
pends 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.
20063025
FIGURE 1. θ_JA vs Copper (1 Ounce) Area for TO-263
package
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