Applications Information: continued
Thermal compound should always be used with high cur-
rent regulators such as these.
IN4002
(optional)
VOUT
The thermal characteristics of an IC depend on the follow-
ing four factors:
VIN
VOUT
VIN
1. Maximum Ambient Temperature TA (¡C)
2. Power dissipation PD (Watts)
C1
CS52015-1
R
C2
1
3. Maximum junction temperature TJ (¡C)
4. Thermal resistance junction to ambient RQJA (C/W)
Adj
These four are related by the equation
R
2
CAdj
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.
Figure 3. Protection diode scheme for Large Output Capacitors.
Output Voltage Sensing
The maximum power dissipation for a regulator is:
Since the CS52015-1 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.
P
D(max)={VIN(max)ÐVOUT(min)}IOUT(max)+VIN(max) Q
I
(2)
where
VIN(max) is the maximum input voltage,
VOUT(min) is the minimum output voltage,
For the adjustable regulator, the best load regulation occurs
when R1 is connected directly to the output pin of the regu-
lator as shown in Figure 3. If R1 is connected to the load,
RC is multiplied by the divider ratio and the effective resis-
tance between the regulator and the load becomes
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.
R1 + R2
RC ´
(
)
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.
R1
RC = conductor parasitic resistance
conductor parasitic
resistance
R
C
VIN
VOUT
VIN
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,
CS52015-1
R
LOAD
R
1
Adj
R
QSA (¡C/W)
R
2
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).
Figure 4. Grounding scheme for the adjustable output regulator to mini-
mize parasitic resistance effects.
The value for RQJC is 3.5ûC/W. For a high current regula-
tor such as the CS52015-1 the majority of the heat is gener-
ated 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
between the heat sink and the package. Once these calcula-
tions are complete, the maximum permissible value of RQJA
can be calculated and the proper heat sink selected. For fur-
ther discussion on heat sink selection, see application note
ÒThermal Management for Linear Regulators.Ó
Calculating Power Dissipation and Heat Sink Requirements
The CS52015-1 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 VOUT on the CS52015-1, and elec-
trical isolation may be required for some applications.
6
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
