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
TPS726126
TPS72615, TPS72616
TPS72618, TPS72625
www.ti.com SLVS403H –MAY 2002–REVISED JUNE 2010
In general, the maximum expected power (P
D(max)
) consumed by a linear regulator is computed as:
(1)
Where:
• V
I(avg)
is the average input voltage.
• V
O(avg)
is the average output voltage.
•
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 ®
qJC
), the case to heatsink ®
qCS
), and the heatsink to ambient
®
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 19 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 19. Thermal Resistances
Equation 2 summarizes the computation:
(2)
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.
Even if no external black body radiator type heatsink is attached to the package, the board on which the
regulator is mounted provides some heatsinking through the pin solder connections. Some packages, like the
DDPAK and SOT223 packages, use a copper plane underneath the package or the circuit board's ground plane
for additional heatsinking to improve their thermal performance. Computer-aided thermal modeling can be used
to compute very accurate approximations of an integrated circuit's thermal performance in different operating
environments (e.g., different types of circuit boards, different types and sizes of heatsinks, and different air flows,
etc.). Using these models, the three thermal resistances can be combined into one thermal resistance between
junction and ambient ®
qJA
). This R
qJA
is valid only for the specific operating environment used in the computer
model.
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