CS5203−1

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7

Calculating Power Dissipation and Heat Sink

Requirements

The CS5203−1 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.

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:

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:

application

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

junction and the surrounding air.

(°C/W)

(°C/W)

3. Thermal Resistance of the Heat Sink to the ambient

These are connected by the equation:

RQJA + RQJC ) RQCS ) RQSA

(3)

result can be substituted in equation (1).

based on an average die size. For a high current regulator

such as the CS5203−1 the majority of the heat is generated

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

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.