Applications Information: continued
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.
Calculating Power Dissipation and Heat Sink Requirements
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.
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 RQJA, the total thermal resistance between the
junction and the surrounding air.
The case is connected to VOUT on the CS5205-X, electrical
isolation may be required for some applications. Thermal
compound should always be used with high current regu-
lators such as these.
1. Thermal Resistance of the junction to case, RQJC (¡C/W)
2. Thermal Resistance of the case to Heat Sink, RQCS (¡C/W)
3. Thermal Resistance of the Heat Sink to the ambient air,
The thermal characteristics of an IC depend on the follow-
ing four factors:
R
QSA (¡C/W)
1. Maximum Ambient Temperature TA (¡C)
2. Power dissipation PD (Watts)
These are connected by the equation:
QJA = RQJC + RQCS + RQSA
3. Maximum junction temperature TJ (¡C)
4. Thermal resistance junction to ambient RQJA (C/W)
R
(3)
The value for RQJA is calculated using equation (3) and the
result can be substituted in equation (1).
These four are related by the equation
The value for RQJC 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 CS5205-X the majority of
the heat is generated in the power transistor section. The
value for RQSA depends on the heat sink type, while RQCS
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 RQJA 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
TJ = TA + PD ´ RQJA
(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)={VIN(max)ÐVOUT(min)}IOUT(max)+VIN(max) Q
I
(2)
where
V
IN(max) is the maximum input voltage,
VOUT(min) is the minimum output voltage,
IOUT(max) is the maximum output current, for the application
IQ is the maximum quiescent current at IOUT(max).
6
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
