CS5205-3 CHERRY [Cherry Semiconductor Corporation], CS5205-3 Datasheet - Page 6

• Manufacturer Part Number: CS5205-3
• Description: 5A Adjustable, and 3.3V and 5V Fixed Linear Regulators
• Manufacturer: CHERRY [Cherry Semiconductor Corporation]

The CS5205-X series of linear regulators includes thermal
shutdown and safe operating area circuitry to protect the
device. High power regulators such as these usually oper-
ate at high junction temperatures so it is important to cal-
culate the power dissipation and junction temperatures
accurately to ensure that an adequate heat sink is used.
The case is connected to V
isolation may be required for some applications. Thermal
compound should always be used with high current regu-
lators such as these.
The thermal characteristics of an IC depend on the follow-
ing four factors:
1. Maximum Ambient Temperature T
2. Power dissipation P
3. Maximum junction temperature T
4. Thermal resistance junction to ambient R
These four are related by the equation
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:
where
V
V
I
I
P
OUT(max)
Q
Calculating Power Dissipation and Heat Sink Requirements
IN(max)
OUT(min)
D(max)
is the maximum quiescent current at I
={V
is the maximum input voltage,
is the maximum output current, for the application
is the minimum output voltage,
IN(max)
T
ÐV
J
= T
OUT(min)
D
A
(Watts)
+ P
OUT
D
}I
on the CS5205-X, electrical
´ R
OUT(max)
QJA
J
(¡C)
A
+V
(¡C)
OUT
Applications Information: continued
IN(max)
QJA
(max).
(C/W)
I
Q
(1)
(2)
6
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
determine R
junction and the surrounding air.
1. Thermal Resistance of the junction to case, R
2. Thermal Resistance of the case to Heat Sink, R
3. Thermal Resistance of the Heat Sink to the ambient air,
These are connected by the equation:
The value for R
The value for R
a given package type based on an average die size. For a
high current regulator such as the CS5205-X the majority of
the heat is generated in the power transistor section. The
value for R
depends on factors such as package type, heat sink inter-
face (is an insulator and thermal grease used?), and the
contact area between the heat sink and the package. Once
these calculations are complete, the maximum permissible
value of R
selected. For further discussion on heat sink selection, see
application note ÒThermal Management for Linear
Regulators.Ó
result can be substituted in equation (1).
R
QSA
(¡C/W)
QJA
QSA
QJA
can be calculated and the proper heat sink
QJC
, the total thermal resistance between the
QJA
depends on the heat sink type, while R
R
is normally quoted as a single figure for
QJA
is calculated using equation (3) and the
= R
QJC
+ R
QCS
+ R
QSA
QJC
QCS
(¡C/W)
(¡C/W)
QCS
(3)