Datasheet

( )
HA JA CH JC
q £ q - q + q
D
J
JA
P
T
T
'
d
A(MAX)J(MAX)J
T-T
'
=
LP38853
SNVS335D –DECEMBER 2006–REVISED APRIL 2013 www.ti.com
The total power dissipation is then:
P
D
= P
D(PASS)
+ P
D(BIAS)
+ P
D(IN)
(11)
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)
) .
(12)
The maximum allowable value for junction to ambient Thermal Resistance, θ
JA
, can be calculated using the
formula:
(13)
Heat-Sinking The PFM Package
The PFM 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 PFM 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
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. (14)
For this equation, θ
JC
is about 3°C/W for a PFM 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 Package
The DDPAK 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 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. Figure 33 shows a curve for the θ
JA
of DDPAK 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 33 shows that increasing the copper area beyond 1 square inch produces very little improvement. The
minimum value for θ
JA
for the DDPAK package mounted to a PCB is 32°C/W.
Figure 34 shows the maximum allowable power dissipation for DDPAK packages for different ambient
temperatures, assuming θ
JA
is 35°C/W and the maximum junction temperature is 125°C.
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