CS5203A−1, CS5203A−2, CS5203A−3, CS5203A−5
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7
Calculating Power Dissipation and Heatsink
Requirements
The CS5203A series of linear regulators includes thermal
shutdown and current limit circuitry to protect the device.
High power regulators such as these usually operate at high
junction temperatures so it is important to calculate the
power dissipation and junction temperatures accurately to
ensure that an adequate Heatsink is used.
The case is connected to V
OUT
on the CS5203A, electrical
isolation may be required for some applications. Thermal
compound should always be used with high current
regulators such as these.
The thermal characteristics of an IC depend on the
following four factors:
1. Maximum Ambient Temperature T
A
(°C)
2. Power dissipation P
D
(Watts)
3. Maximum junction temperature T
J
(°C)
4. Thermal resistance junction to ambient R
JA
(°C/W)
These four are related by the equation
T
J
T
A
P
D
R
JA
(1)
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:
P
D(max)
{
V
IN(max)
V
OUT(min)
}
I
OUT(max)
V
IN(max)
I
Q
(2)
where:
V
IN(max)
is the maximum input voltage,
V
OUT(min)
is the minimum output voltage,
I
OUT(max)
is the maximum output current, for the
application
I
Q
is the maximum quiescent current at I
OUT(max)
.
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. Like series
electrical resistances, these resistances are summed to
determine R
JA
, the total thermal resistance between the
junction and the surrounding air.
1. Thermal Resistance of the junction−to−case, R
JC
(°C/W)
2. Thermal Resistance of the case to Heatsink, R
CS
(°C/W)
3. Thermal Resistance of the Heatsink to the ambient
air, R
SA
(°C/W)
These are connected by the equation:
R
JA
R
JC
R
CS
R
SA
(3)
The value for R
JA
is calculated using equation (3) and the
result can be substituted in equation (1).
The value for R
JC
is normally quoted as a single figure
for a given package type based on an average die size. For
a high current regulator such as the CS5203A the majority
of the heat is generated in the power transistor section. The
value for R
SA
depends on the Heatsink type, while R
CS
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
JA
can be calculated and the proper Heatsink selected.
For further discussion on Heatsink selection, see application
note “Thermal Management,” document number
AND8036/D, available through the Literature Distribution
Center or via our website at http://onsemi.com.
