MIC4680 datasheet(13/24 Pages) MICROCHIP

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DS20006623A-page 13

MIC4680

5.2

Thermal Considerations

The MIC4680 SuperSwitcher features the power

SOIC-8. This package has a standard 8-pin

small-outline package profile but with much higher

power dissipation than a standard SOIC-8. The

MIC4680 SuperSwitcher is the first DC-to-DC

converter to take full advantage of this package.

The reason that the power SOIC-8 has higher power

dissipation (lower thermal resistance) is that pins 5

though 8 and the die-attach paddle are a single piece

of metal. The die is attached to the paddle with

thermally conductive adhesive. This provides a low

thermal resistance path from the junction of the die to

the ground pins. This design significantly improves

package power dissipation by allowing excellent heat

transfer through the ground leads to the printed circuit

board.

One of the limitation of the maximum output current on

any MIC4680 design is the junction-to-ambient thermal

resistance (θJA) of the design (package and ground

plane).Examining θJA in more detail:

EQUATION 5-3:

θJC is a relatively constant 20°C/W for a power SOIC-8.

θCA is dependent on layout and is primarily governed

by the connection of pins 5 though 8 to the ground

plane. The purpose of the ground plane is to function

as a heat sink.

θJA is ideally 63°C/W but will vary depending on the

size of the ground plane to which the power SOIC-8 is

attached.

5.3

Determining Ground-Plane

Heat-Sink Area

There are two methods of determining the minimum

ground plane area required by the MIC4680.

5.3.1

QUICK METHOD

Make sure that MIC4680 pins 5 though 8 are

connected to a ground plane with a minimum area of

6cm2. This ground plane should be as close to the

MIC4680 as possible. The area maybe distributed in

any shape around the package or on any PCB layer as

long as there is good thermal contact to pins 5 though

8. This ground plane area is more than sufficient for

most designs.

FIGURE 5-1:

Power SOIC-8 Cross

Section.

5.3.2

MINIMUM COPPER/MAXIMUM

CURRENT METHOD

Using Figure 5-2, for a given input voltage range,

determine the minimum ground-plane heat-sink area

required for the application’s maximum output current.

Figure 5-2 assumes a constant die temperature of

75°C above ambient.

FIGURE 5-2:

Output Current vs. Ground

Plane Area.

When designing with the MIC4680, it is a good practice

to connect pins 5 through 8 to the largest ground plane

that is practical for the specific design.

JA JC CA

Where:

θJC = Junction-to-case thermal resistance.

θCA = Case-to-ambient thermal resistance.

BIEN

Printed Circuit Board

Ground Plane

Heat Sink Area

SOIC-8

0.5

1.0

1.5

AREA (cm2)

12V

34V

24V

A =50°C

Minimum Current Limit = 1.3A

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