CS5253B鈭?
R
DIS
+5.0 V
+3.3 V
+
+
V
CONTROL
V
POWER
V
OUT
V
SENSE
GND
+
+Load
CS5253B鈭?
10
mF
100
mF
Local
Connections
33
mF
Remote
Connections
+
Optional
GND
R
DIS
鈭扡oad
Figure 16. Remote Sense
Calculating Power Dissipation and Heatsink
Requirements
High power regulators such as the CS5253B鈭? usually
operate at high junction temperatures. Therefore, it is
important to calculate the power dissipation and junction
temperatures accurately to ensure that an adequate heatsink
is used. Since the package tab is connected to V
OUT
on the
CS5253B鈭?, electrical isolation may be required for some
applications. Also, as with all high power packages, thermal
compound in necessary to ensure proper heat flow. For
added safety, this high current LDO includes an internal
thermal shutdown circuit
The thermal characteristics of an IC depend on the
following four factors: junction temperature, ambient
temperature, die power dissipation, and the thermal
resistance from the die junction to ambient air. The
maximum junction temperature can be determined by:
TJ(max)
+
TA(max)
)
PD(max)
R
qJA
A heatsink effectively increases the surface area of the
package to improve the flow of heat away from the IC and
into the surrounding air. Each material in the heat flow path
between the IC and the outside environment has a thermal
resistance which is measured in degrees per watt. Like series
electrical resistances, these thermal resistances are summed
to determine the total thermal resistance between the die
junction and the surrounding air, R
qJA
. This total thermal
resistance is comprised of three components. These resistive
terms are measured from junction鈭抰o鈭抍ase (R
qJC
),
case鈭抰o鈭抙eatsink (R
qCS
), and heatsink鈭抰o鈭抋mbient air
(R
qSA
). The equation is:
R
qJA
+
R
qJC
)
R
qCS
)
R
qSA
The maximum ambient temperature and the power
dissipation are determined by the design while the
maximum junction temperature and the thermal resistance
depend on the manufacturer and the package type. The
maximum power dissipation for a regulator is:
PD(max)
+
(VIN(max)
*
VOUT(min))IOUT(max)
)
VIN(max) IIN(max)
The value for R
qJC
is 2.5擄C/watt for the CS5253B鈭? in the
D2PAK鈭? package. For a high current regulator such as the
CS5253B鈭? the majority of heat is generated in the power
transistor section. The value for R
qSA
depends on the
heatsink type, while the R
qCS
depends on factors such as
package type, heatsink interface (is an insulator and thermal
grease used?), and the contact area between the heatsink and
the package. Once these calculations are complete, the
maximum permissible value of R
qJA
can be calculated and
the proper heatsink selected. For further discussion on
heatsink selection, see our application note 鈥淭hermal
Management,鈥?document number AND8036/D.
http://onsemi.com
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