CS8481 datasheet(5/6 Pages) ONSEMI

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CS8481

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ENABLE

The ENABLE function controls VOUT1. When ENABLE

is high, VOUT1 is on. When ENABLE is low, VOUT1 is off.

Calculating Power Dissipation in a

Dual Output Linear Regulator

The maximum power dissipation for a dual output

regulator (Figure 3) 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

application,

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

application, and

IQ is the quiescent current the regulator consumes at both

IOUT1(max) and IOUT2(max).

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

permissible value of RqJA can be calculated:

QJA +

150

°C * TA

(2)

The value of RqJA can 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 3. Dual Output Regulator With Key

Performance Parameters Labeled.

SMART

REGULATOR

Control

Features

VOUT1

IOUT1

VOUT2

IOUT2

VIN

IIN

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 RqJA:

QJA + RQJC ) RQCS ) RQSA

(3)

where:

RqJC = the junction−to−case thermal resistance,

RqCS = the case−to−heatsink thermal resistance, and

RqSA = the heatsink−to−ambient 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 interface

between them. These values appear in heat sink data sheets

of heat sink manufacturers.

Figure 4. Test and Application Circuit

* C1 required if regulator is located far from power supply filter.

** C2 and C3 required for stability. Capacitor must operate at minimum temperature expected during system operations.

ENABLE

VOUT2

VIN

VOUT1

CS8481

GND

Load

VBATT

VCC

I/O

GND

C3**

C1*

C2**

0.1

ESR < 8

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