CS8321 datasheet(4/6 Pages) CHERRY

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Circuit Description and Application Notes: continued

The maximum power dissipation for a single output regu-

lator (Figure 3) is:

PD(max)=

(VIN(max)ÐVOUT(min))IOUT(max)+VIN(max)IQ

(1)

where:

VIN(max) is the maximum input voltage,

VOUT(min) is the minimum output voltage,

IOUT(max) is the maximum output current for the applica-

tion, and

IQ is the quiescent current the regulator consumes at

IOUT(max).

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.

Figure 4: Single output regulator with key performance parameters

labeled.

A heatsink 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 too is a function of package type. RQCS and RQSA

are functions of the package type, heatsink and the inter-

face between them. These values appear in heatsink data

sheets of heatsink manufacturers.

Heatsinks

VIN

CS8321

VOUT

IOUT

IIN

150¡C - TA

Calculating Power Dissipation

in a Single Output Linear Regulator

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