CS8251ETHA7 데이터 시트보기 (PDF) - ON Semiconductor

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CS8251ETHA7

10V/5V Low Dropout Dual Regulator with Independent Output Enables

ON Semiconductor 

Application Notes: continued

Calculating Power Dissipation

in a Dual Output Linear Regulator

The maximum power dissipation for a dual output regu-

lator (Figure 1) is

PD(max) = {VIN(max)–VOUT1(min)}IOUT1(max)+

{VIN(max)–VOUT2(min)}IOUT2(max)+VIN(max)IQ (1)

Where

VIN(max) is the maximum input voltage,

VOUT1(min) is the minimum output voltage from VOUT1,

VOUT2(min) is the minimum output voltage from VOUT2,

IOUT1(max) is the maximum output current, for the appli-

cation

IOUT2(max) is the maximum output current, for the appli-

cation

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 RΘJA can be calculated:

RΘJA =

150°C - TA

PD

(2)

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.

IIN

VIN

Smart

Regulator

}Control

Features

IQ

IOUT1

IOUT2

VOUT1

VOUT2

Figure 1: Dual output regulator with key performance parameters

labeled.

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.

RΘJA = RΘJC + RΘCS + RΘSA

(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 inter-

face between them. These values appear in heat sink data

sheets of heat sink manufacturers.

5

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