CS9201 datasheet(5/8 Pages) ONSEMI

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CS9201

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CALCULATING POWER DISSIPATION IN A

SINGLE OUTPUT LINEAR REGULATOR

The maximum power dissipation for a single output

regulator (Figure 10) is:

PD(max) + {VIN(max) * VOUT(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

application, and

IQ is the quiescent current the regulator consumes at

IOUT(max).

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

permissible value of RΘJA can be calculated:

QJA +

150

°C * TA

(2)

Figure 10. Single output regulator with key

performance parameters labeled.

IIN

IOUT

CS9201

VOUT

VIN

The value of RΘJA can then be compared with those in the

package section of the data sheet. Those packages with

RΘJA ’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.

HEAT SINKS

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 RΘJA:

QJA + RQJC ) RQCS ) RQSA

(3)

where:

RΘJC = the junction–to–case thermal resistance,

RΘCS = the case–to–heatsink thermal resistance, and

RΘSA = the heatsink–to–ambient thermal resistance.

RΘJC appears in the package section of the data sheet.

Like RΘJA, it too is a function of package type. RΘCS and

RΘSA are functions of the package type, heatsink and the

interface between them. These values appear in heat sink

data sheets of heat sink manufacturers.

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