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CS5207-2GT3

7.0 A, 1.5 V Fixed Linear Regulator

ON Semiconductor 

CS5207−2

Calculating Power Dissipation and Heat Sink

Requirements

The CS5207−2 includes thermal shutdown and safe

operating area 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 CS5207−2,

electrical isolation may be required for some applications.

Thermal compound should always be used with high current

regulators such as these.

The thermal characteristics of an IC depend on the

following 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 four are related by the equation

TJ + TA ) PD RQJA

(1)

The maximum ambient temperature and the power

dissipation 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.

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)

These are connected by the equation:

RQJA + RQJC ) RQCS ) RQSA

(3)

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

the result can be substituted in equation (1).

RΘJC 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

RΘSA depends on the heat sink type, while RΘCS depends 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

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,” document number

AND8036/D, available through the Literature Distribution

Center or via our website at http://onsemi.com.

ADDITIONAL ORDERING INFORMATION

Orderable Part

Number

Type

CS5207−2GT3

7.0 A, 1.5 V Output

Description

TO−220 THREE LEAD, STRAIGHT

http://onsemi.com

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