Datasheet
T
HA
d T
JA
- (T
CH
+ T
JC
)
d
'T
J
P
D
T
JA
'T
J
= T
J(MAX)
- T
A(MAX)
LP38858
SNVS462D –OCTOBER 2006–REVISED APRIL 2013
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The third part is the power that is dissipated in portions of the output stage circuitry, and can be determined with
the formula:
P
D(IN)
= V
IN
× I
GND(IN)
where
• I
GND(IN)
is the portion of the operating ground current of the device that is related to V
IN
(4)
The total power dissipation is then:
P
D
= P
D(PASS)
+ P
D(BIAS)
+ P
D(IN)
(5)
The maximum allowable junction temperature rise (ΔT
J
) depends on the maximum anticipated ambient
temperature (T
A
) for the application, and the maximum allowable operating junction temperature (T
J(MAX)
) .
(6)
The maximum allowable value for junction to ambient Thermal Resistance, θ
JA
, can be calculated using the
formula:
(7)
Heat-Sinking the TO-220 Package
The TO-220-5 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. If the needed θ
JA
, as calculated above, is greater than or
equal to 60°C/W then no additional heat-sinking is required since the package can safely dissipate the heat and
not exceed the operating T
J(MAX)
. If the needed θ
JA
is less than 60°C/W then additional heat-sinking is needed.
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 heat-sink to be used in the application should have a heat-sink to ambient thermal resistance, θ
HA
:
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 heart-sink
• θ
JC
is the thermal resistance from the junction to the surface of the case (8)
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, insulator, etc. θ
CH
varies between 1.5°C/W to 2.5°C/W. Consult the heat-sink manufacturer
datasheet for details and recommendations.
Heat-Sinking the DDPAK/TO-263 Package
The DDPAK/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 DDPAK/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. 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.
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