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Fabricante

CS5207-1

7A Adjustable Linear Regulator

Cherry semiconductor 

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, RC is multiplied by the divider

ratio and the effective resistance between the regulator and

the load becomes

( ) RC × R1 + R2

R1

RC = conductor parasitic resistance

VIN

VIN

VOUT

conductor parasitic

RC resistance

CS5207-1

R1

Adj

RLOAD

R2

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:

PD(max)={VIN(max)–VOUT(min)}IOUT(max)+VIN(max)IQ

(2)

where

VIN(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).

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 RΘJA, the total thermal resistance between the

junction and the surrounding air.

Figure 3. Grounding scheme for the adjustable output regulator to min-

imize parasitics.

1. Thermal Resistance of the junction to case, RΘJC (°C/W)

2. Thermal Resistance of the case to Heat Sink, RΘCS (°C/W)

3. Thermal Resistance of the Heat Sink to the ambient air,

RΘSA (°C/W)

Calculating Power Dissipation and Heat Sink Requirements

The CS5207-1 linear regulator includes thermal shutdown

and safe operating area circuitry to protect the device.

High power regulators such as this 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 CS5207-1, 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 TA (°C)

2. Power dissipation PD (Watts)

3. Maximum junction temperature TJ (°C)

4. Thermal resistance junction to ambient RΘJA (C/W)

These are connected by the equation:

RΘJA = RΘJC + RΘCS + RΘSA

(3)

The value for RΘJA is calculated using equation (3) and the

result can be substituted in equation (1).

The value for RΘJC 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 CS5207-1 the majority of

the heat is generated in the power transistor section. The

value for RΘSA depends on the heat sink type, while RΘCS

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 RΘJA can be calculated and the proper heat sink

selected. For further discussion on heat sink selection, see

application note “Thermal Management for Linear

Regulators.”

These four are related by the equation

TJ = TA + PD × RΘJA

(1)

5

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