CS5207-3
Applications Information: continued
determine R
QJA
, the total thermal resistance between the
junction and the surrounding air.
1. Thermal Resistance of the junction to case, R
QJC
(¡C/W)
R
LOAD
V
IN
V
IN
V
OUT
R
C
conductor parasitic
resistance
CS5207-3
Gnd
2. Thermal Resistance of the case to Heat Sink, R
QCS
(¡C/W)
3. Thermal Resistance of the Heat Sink to the ambient air,
R
QSA
(¡C/W)
These are connected by the equation:
R
QJA
= R
QJC
+ R
QCS
+ R
QSA
(3)
Figure 2. Grounding scheme for the output regulator to minimize para-
sitics.
The value for R
QJA
is calculated using equation (3) and the
result can be substituted in equation (1).
R
QJC
is 1.6¡C/Watt for the CS5207-3. For a high current
regulator such as the CS5207-3, the majority of the heat is
generated in the power transistor section. The value for
R
QSA
depends on the heat sink type, while R
QCS
depends
on factors such as package type, heat sink interface (is an
insulator and thermal grease used?), and the contact area
between the heat sink and the package. Once these calcula-
tions are complete, the maximum permissible value of
R
QJA
can be calculated and the proper heat sink selected.
For further discussion on heat sink selection, see applica-
tion note ÒThermal Management for Linear Regulators.Ó
Calculating Power Dissipation and Heat Sink Requirements
The CS5207-3 linear regulator includes thermal shutdown
and safe operating area circuitry to protect the device.
High power regulators such as this usually operate at high
junction temperatures so it is important to calculate the
power dissipation and junction temperatures accurately to
ensure that an adequate heat sink is used.
The case is connected to V
OUT
on the CS5207-3, and electri-
cal isolation may be required for some applications.
Thermal compound should always be used with high cur-
rent regulators such as these.
The thermal characteristics of an IC depend on the follow-
ing 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
QJA
(C/W)
These four are related by the equation
T
J
= T
A
+ P
D
´
R
QJA
(1)
The maximum ambient temperature and the power dissi-
pation 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)
ÐV
OUT(min)
}I
OUT(max)
+V
IN(max)
I
Q
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 heat sink 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
4
(2)
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
