CS5205A-1
Applications Information: continued
Best load regulation occurs when R1 is connected directly
to the output pin of the regulator as shown in Figure 3. If
R1 is connected to the load, R
C
is multiplied by the divider
ratio and the effective resistance between the regulator and
the load becomes
R
C
´
R1 + R2
R1
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.
V
IN
V
OUT
R
C
conductor parasitic
resistance
(2)
(
)
R
C
= conductor parasitic resistance
CS5205A-1
R
1
Adj
R
LOAD
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)
R
2
3. Thermal Resistance of the Heat Sink to the ambient air,
R
QSA
(¡C/W)
These are connected by the equation:
Figure 3. Grounding scheme for the adjustable output regulator to min-
imize parasitics.
R
QJA
= R
QJC
+ R
QCS
+ R
QSA
(3)
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 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 CS5205A-1 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 calculations are complete, the maximum per-
missible value of R
QJA
can be calculated and the proper
heat sink selected. For further discussion on heat sink
selection, see application note ÒThermal Management for
Linear Regulators.Ó
Calculating Power Dissipation and Heat Sink Requirements
The CS5205A-1 linear regulator includes thermal shut-
down 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
OUT
on the CS5205A-1, electrical
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
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.
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
