CS5203-3
Applications Information: continued
The maximum power dissipation for a regulator is:
R
C
V
IN
V
IN
V
OUT
conductor
parasitic resistance
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,
(2)
CS5203-3
GND
R
LOAD
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.
Figure 2. Conductor parasitic resistance effects can be minimized with
the above grounding scheme for fixed output regulators.
Calculating Power Dissipation and Heat Sink Requirements
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
QJA
, the total thermal resistance between the
junction and the surrounding air.
1. Thermal Resistance of the junction to case, R
QJC
(¡C/W)
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)
The CS5203-3 linear regulator 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 heat sink is used.
The case is connected to V
OUT
on the CS5203-3, 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 value for R
QJA
is calculated using equation (3) and the
result can be substituted in equation (1).
The value for R
QJC
is 3.5ûC/W. For a high current regula-
tor such as the CS5203-3 the majority of the heat is generat-
ed in the power transistor section. The value for R
QSA
depends on the heat sink type, while R
QCS
depends on fac-
tors such as package type, heat sink interface (is an insula-
tor 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
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.
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
