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SLVS351I – SEPTEMBER 2002 – REVISED MAY 2006
THERMAL INFORMATION
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)
)
consumed by a linear regulator is computed as
P
D
max
+
V
IN(avg)
*
V
OUT(avg)
I
OUT(avg)
)V
IN(avg)
I
(Q)
(4)
dissipation. The temperature rise is computed by
multiplying the maximum expected power dissipation
by the sum of the thermal resistances between the
junction and the case (R
θJC
), the case to heatsink
(R
θCS
), and the heatsink to ambient (R
θSA
). Thermal
resistances are measures of how effectively an
object dissipates heat. Typically, the larger the
device, the more surface area available for power
dissipation and the lower the object's thermal
resistance.
illustrates these thermal resistances for (a)
a SOT223 package mounted in a JEDEC low-K
board, and (b) a DDPAK package mounted on a
JEDEC high-K board.
summarizes the computation:
T
J
+
T
)
P
D
max x R
)
R
)
R
A
θJC
θCS
θSA
(5)
where:
•
V
IN(avg)
is the average input voltage.
•
V
OUT(avg)
is the average output voltage.
•
I
OUT(avg)
is the average output current.
•
I
(Q)
is the quiescent current.
For most TI LDO regulators, the quiescent current is
insignificant compared to the average output current;
therefore, the term V
IN(avg)
×
I
(Q)
can be neglected.
The operating junction temperature is computed by
adding the ambient temperature (T
A
) and the
increase in temperature due to the regulator's power
A
CIRCUIT BOARD COPPER AREA
C
B
B
R
θCS
A
C
R
θSA
R
θJC
The R
θJC
is specific to each regulator as determined
by its package, lead frame, and die size provided in
the regulator's data sheet. The R
θSA
is a function of
the type and size of heatsink. For example,
black
body radiator
type heatsinks can have R
θCS
values
ranging from 5°C/W for very large heatsinks to
50°C/W for very small heatsinks. The R
θCS
is a
function of how the package is attached to the
heatsink. For example, if a thermal compound is
used to attach a heatsink to a SOT223 package,
R
θCS
of 1°C/W is reasonable.
T
J
A
B
T
C
SOT223 Package
(a)
T
A
DDPAK Package
(b)
C
Figure 24. Thermal Resistances
10
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