Applications Information: continued
The maximum power dissipation for a regulator is:
PD(max)={VIN(max)ÐVOUT(min)}IOUT(max)+VIN(max) Q
where
conductor
parasitic resistance
R
I
(2)
C
VIN
VOUT
VIN
CS5201-3
RLOAD
V
V
IN(max) is the maximum input voltage,
OUT(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).
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 RQJA, the total thermal resistance between the
junction and 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
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 CS5201-3 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.
R
QSA (¡C/W)
The case is connected to VOUT on the CS5201-3, and electri-
cal isolation may be required for some applications.
Thermal compound should always be used with high cur-
rent regulators such as these.
These are connected by the equation:
QJA = RQJC + RQCS + RQSA
R
(3)
The value for RQJA is calculated using equation (3) and the
result can be substituted in equation (1).
The thermal characteristics of an IC depend on the follow-
ing four factors:
The value for RQJC is 3.5ûC/W. For a high current regula-
tor such as the CS5201-3 the majority of the heat is generat-
ed in the power transistor section. The value for RQSA
depends on the heat sink type, while RQCS depends on fac-
tors such as package type, heat sink interface (is an insula-
tor and thermal grease used?), and the contact area
1. Maximum Ambient Temperature TA (¡C)
2. Power dissipation PD (Watts)
3. Maximum junction temperature TJ (¡C)
4. Thermal resistance junction to ambient RQJA (C/W)
between the heat sink and the package. Once these calcula-
tions are complete, the maximum permissible value of
These four are related by the equation
R
QJA can be calculated and the proper heat sink selected.
TJ = TA + PD ´ RQJA
(1)
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 ]
