LP38855 Datasheet, PDF (11/14 Pages) National Semiconductor (TI) – 1.5A Fast-Response High-Accuracy LDO Linear Regulator with Enable

The second part is the power that is dissipated in the bias and
control circuitry, and can be determined with the formula:
P_D(BIAS) = V_BIAS × I_GND(BIAS)
(2)
where I_GND(BIAS) is the portion of the operating ground current
of the device that is related to V_BIAS.
The third part is the power that is dissipated in portions of the
output stage circuitry, and can be determined with the formu-
la:
P_D(IN) = V_IN × I_GND(IN)
(3)
where I_GND(IN) is the portion of the operating ground current
of the device that is related to V_IN.
The total power dissipation is then:
P_D = P_D(PASS) + P_D(BIAS) + P_D(IN)
(4)
The maximum allowable junction temperature rise (ΔT_J) de-
pends on the maximum anticipated ambient temperature (T_A
(MAX)^{) for the application, and the maximum allowable oper-}
ating junction temperature (T_J(MAX)):
(5)
The maximum allowable value for junction to ambient Ther-
mal Resistance, θ_JA, can be calculated using the formula:
(6)
Heat-Sinking The TO-220 Package
The TO-220 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 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 TO-263 package.
The heat-sink to be used in the application should have a
heat-sink to ambient thermal resistance, θ_HA:
(7)
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 heat sink, and θ_JC is the thermal
resistance from the junction to the surface of the case.
For this equation, θ_JC is about 3°C/W for a TO-220 package.
The value for θ_CH depends on method of attachment, insula-
tor, etc. θ_CH varies between 1.5°C/W to 2.5°C/W. Consult the
heat-sink manufacturer datasheet for details and recommen-
dations.
Heat-Sinking The TO-263 Package
The TO-263 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 TO-263 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. 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.
20202625
FIGURE 1. θ_JA vs Copper (1 Ounce) Area for the TO-263
package
As shown in Figure 1, 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 2 shows the maximum allowable power dissipation for
TO-263 packages for different ambient temperatures, assum-
ing θ_JA is 35°C/W and the maximum junction temperature is
125°C.
20202626
FIGURE 2. Maximum Power Dissipation vs Ambient
Temperature for TO-263 package
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