CS8126 datasheet(7/9 Pages) CHERRY

7 / 9 page

Application Notes: continued

Once the value of PD(max) is known, the maximum permis-

sible value of RQJA can be calculated:

RQJA =

(2)

The value of RQJA can then be compared with those in

the package section of the data sheet. Those packages with

RQJA's less than the calculated value in equation 2 will keep

the die temperature below 150¡C.

In some cases, none of the packages will be sufficient to

dissipate the heat generated by the IC, and an external

heatsink will be required.

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 will have a thermal resistance. Like

series electrical resistances, these resistances are summed

to determine the value of RQJA.

RQJA = RQJC + RQCS + RQSA

(3)

where:

RQJC = the junctionÐtoÐcase thermal resistance,

RQCS = the caseÐtoÐheatsink thermal resistance, and

RQSA = the heatsinkÐtoÐambient thermal resistance.

RQJC appears in the package section of the data sheet. Like

RQJA, it is a function of package type. RQCS and RQSA are

functions of the package type, heatsink and the interface

between them. These values appear in heat sink data

sheets of heat sink manufacturers.

150¡C - TA

Heat Sinks

VIN

Smart

Regulator

VOUT

IOUT

IIN

Control

Features

}

Figure 1. Single output regulator with key performance parameters

labeled.

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