Datasheet

P
D
max +
ǒ
V
I(avg)
* V
O(avg)
Ǔ
I
O(avg)
) V
I(avg)
x I
(Q)
A
B
C
T
J
A
R
θ
JC
T
C
B
R
θ
CS
T
A
C
R
θ
SA
(a)
(b)
DDPAK Package
SOT223 Package
CIRCUIT BOARD COPPER AREA
B
A
C
T
J
+ T
A
) P
D
max x
ǒ
R
θJC
) R
θCS
) R
θSA
Ǔ
TPS72501
TPS72515, TPS72516
TPS72518, TPS72525
www.ti.com SLVS341E –MAY 2002–REVISED JUNE 2010
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:
(3)
Where:
• V
I(avg)
is the average input voltage.
• V
O(avg)
is the average output voltage.
• I
O(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
I(avg)
x 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 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
qJC
), the case to heatsink (R
qCS
), and the heatsink to ambient
(R
qSA
). 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.
Figure 20 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.
Figure 20. Thermal Resistances
Equation 4 summarizes the computation:
(4)
The R
qJC
is specific to each regulator as determined by its package, lead frame, and die size provided in the
regulator's data sheet. The R
qSA
is a function of the type and size of heatsink. For example, black body radiator
type heatsinks can have R
qCS
values ranging from 5°C/W for very large heatsinks to 50°C/W for very small
heatsinks. The R
qCS
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
qCS
of 1°C/W is reasonable.
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