RT9199GS datasheet(10/13 Pages) RICHTEK

10 / 13 page

RT9199

DS9199-07

September 2007

www.richtek.com

The maximum power dissipation depends on the thermal

resistance of IC package, PCB layout, the rate of

surroundings airflow and temperature difference between

junction to ambient. The maximum power dissipation can

be calculated by following formula:

PD(MAX) = ( TJ(MAX)

− TA ) / θJA

Where TJ(MAX) is the maximum operation junction

temperature 125

°C, TA is the ambient temperature and the

θJA is the junction to ambient thermal resistance. The

junction to ambient thermal resistance for SOP-8 package

(Exposed Pad) is 86

°C/W, on standard JEDEC 51-7 (4

layers, 2S2P) thermal test board. The maximum power

dissipation at TA = 25

°C can be calculated by following

formula:

PD(MAX) = (125

°C − 25°C) / 86°C/W = 1.163W

Figure 8 shows the package sectional drawing of SOP-8

(Exposed Pad). Every package has several thermal

dissipation paths. As show in Figure 9, the thermal

resistance equivalent circuit of SOP-8 (Exposed Pad). The

path 2 is the main path due to these materials thermal

conductivity. We define the exposed pad is the case point

of the path 2.

Figure 8. SOP-8 (Exposed Pad) Package Sectional

Drawing

Ambient

Molding Compound

Gold Line

Lead Frame

Die Pad

Case (Exposed Pad)

PCB

The thermal resistance

θJA of SOP-8 (Exposed Pad) is

determined by the package design and the PCB design.

However, the package design has been decided. If possible,

it’ s useful to increase thermal performance by the PCB

design. The thermal resistance can be decreased by

adding copper under the expose pad of SOP-8 package.

Figure 10 show the relation between thermal resistance

θJA and copper area on a standard JEDEC 51-7 (4 layers,

2S2P) thermal test board at TA = 25

°C. We have to consider

the copper couldn’ t stretch infinitely and avoid the tin

overflow. We use the “Dog-Bone” copper patterns on the

top layer as Figure 11.

100

2030

405060

7080

Copper Area (mm

Figure 10. Relation Between Thermal Resistance

JA and

Copper Area

Figure 11. Dog-Bone Layout

Exposed Pad

W≦2.28mm

Figure 9. Thermal Resistance Equivalent Circuit

Junction

DIE

DIE-ATTACH RDIE-PAD

GOLD-LINE

LEAD FRAME

Case

(Exposed Pad)

PCB

Ambient

MOLDING-COMPOUND

path 1

path 2

path 3

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