Applications Information: continued
P
D(max)={VIN(max)ÐVOUT(min)}IOUT(max)+VIN(max) Q
I
(2)
Output Voltage Sensing
where
Since the CS5207-2 is a three terminal regulator, it is not
possible to provide true remote load sensing. Load regula-
tion is limited by the resistance of the conductors connect-
ing the regulator to the load. For best results the fixed reg-
ulators should be connected as shown in Figure 2.
VIN(max) is the maximum input voltage,
V
OUT(min) is the minimum output voltage,
IOUT(max) is the maximum output current, for the application
Q is the maximum quiescent current at IOUT(max).
I
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.
conductor parasitic
resistance
R
C
VIN
VOUT
VIN
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.
CS5207-2
R
LOAD
Gnd
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,
R
QSA (¡C/W)
Figure 2. Conductor parasitic resistance effects can be minimized with
the above grounding scheme for fixed output regulators.
These are connected by the equation:
QJA = RQJC + RQCS + RQSA
R
(3)
Calculating Power Dissipation and Heat Sink Requirements
The value for RQJA is calculated using equation (3) and the
result can be substituted in equation (1).
The CS5207-2 includes thermal shutdown and safe operat-
ing area circuitry to protect the device. High power regu-
lators such as these usually operate at high junction tem-
peratures so it is important to calculate the power dissipa-
tion and junction temperatures accurately to ensure that an RQSA depends on the heat sink type, while RQCS depends
adequate heat sink is used.
RQJC is 1.6¡C/Watt for the CS5207-2. For a high current
regulator such as the CS5207-2 the majority of the heat is
generated in the power transistor section. The value for
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 calcula-
tions are complete, the maximum permissible value of
The case is connected to VOUT on the CS5207-2, electrical
isolation may be required for some applications. Thermal
compound should always be used with high current regu-
lators such as these.
RQJA 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 thermal characteristics of an IC depend on the follow-
ing four factors:
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)
These four are related by the equation
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:
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
