Datasheet

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THERMAL INFORMATION
T
J
A
R
θ
JC
T
C
B
R
θ
CS
T
A
C
R
θ
SA
SOT223 Package
CIRCUIT BOARD COPPER AREA
B
A
C
P
D
max +
ǒ
V
IN(avg)
* V
OUT(avg)
Ǔ
I
OUT(avg)
) V
I(avg)
I
Q
T
J
+ T
A
) P
D
max
ǒ
R
θJC
) R
θCS
) R
θSA
Ǔ
(5)
TPS794xx
SLVS349E – NOVEMBER 2001 – REVISED DECEMBER 2005
The amount of heat that an LDO linear regulator
generates is directly proportional to the amount of
power it dissipates during operation. All integrated
circuits have a maximum allowable junction
temperature (T
J
max) above which normal operation
is not assured. A system designer must design the
operating environment so that the operating junction
temperature (T
J
) does not exceed the maximum
junction temperature (T
J
max). The two main
environmental variables that a designer can use to
improve thermal performance are air flow and
external heatsinks. The purpose of this information is
to aid the designer in determining the proper
operating environment for a linear regulator that is
operating at a specific power level.
In general, the maximum expected power (P
D
max)
Figure 24. Thermal Resistances
consumed by a linear regulator is computed as
shown in Equation 4 :
Equation 5 summarizes the computation:
(4)
where:
The R
Θ JC
is specific to each regulator as determined
by its package, lead frame, and die size provided in
• V
IN(avg)
is the average input voltage
the regulator's data sheet. The R
Θ SA
is a function of
• V
OUT(avg)
is the average output voltage
the type and size of heatsink. For example, black
• I
OUT(avg)
is the average output current
body radiator type heatsinks can have R
Θ CS
values
• I
Q
is the quiescent current
ranging from 5 ° C/W for very large heatsinks to
50 ° C/W for very small heatsinks. The R
Θ CS
is a
For most TI LDO regulators, the quiescent current is
function of how the package is attached to the
insignificant compared to the average output current;
heatsink. For example, if a thermal compound is
therefore, the term V
IN(avg)
x I
Q
can be neglected. The
used to attach a heatsink to a SOT223 package,
operating junction temperature is computed by
R
Θ CS
of 1 ° C/W is reasonable.
adding the ambient temperature (T
A
) and the
increase in temperature due to the regulator's power
Even if no external black body radiator type heatsink
dissipation. The temperature rise is computed by
is attached to the package, the board on which the
multiplying the maximum expected power dissipation
regulator is mounted provides some heatsinking
by the sum of the thermal resistances between the
through the pin solder connections. Some packages,
junction and the case (R
Θ JC
), the case to heatsink
like the DDPAK and SOT223 packages, use a
(R
Θ CS
), and the heatsink to ambient (R
Θ SA
). Thermal
copper plane underneath the package or the circuit
resistances are measures of how effectively an
board ground plane for additional heatsinking to
object dissipates heat. Typically, the larger the
improve their thermal performance. Computer-aided
device, the more surface area available for power
thermal modeling can be used to compute very
dissipation and the lower the object's thermal
accurate approximations of an integrated circuit's
resistance.
thermal performance in different operating
environments (for example, different types of circuit
Figure 24 illustrates these thermal resistances for a
boards, different types and sizes of heatsinks,
SOT223 package mounted in a JEDEC low-K board.
different air flows, etc.). Using these models, the
three thermal resistances can be combined into one
thermal resistance between junction and ambient
(R
Θ JA
). This R
Θ JA
is valid only for the specific
operating environment used in the computer model.
10
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