RT8294 datasheet(12/15 Pages) RICHTEK

12 / 15 page

RT8294

DS8294-02

March 2011

www.richtek.com

Figure 7. Derating Curves for RT8294 Package

(a) Copper Area = (2.3 x 2.3) mm

θJA = 75°C/W

(b) Copper Area = 10mm

θJA = 64°C/W

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

100

125

Ambient Temperature (

°C)

Copper Area

70mm

50mm

30mm

10mm

Min.Layout

SOP-8

Four-Layer PCB

Thermal Considerations

For continuous operation, do not exceed absolute

maximum operation junction temperature 125

°C. The

maximum power dissipation depends on the thermal

resistance of IC package, PCB layout, the rate of

surroundings airflow and temperature difference between

junctions 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, TA is the ambient temperature and the

θJA

is the junction to ambient thermal resistance.

For recommended operating conditions specification of

RT8294, the maximum junction temperature is 125

°C. The

junction to ambient thermal resistance

θJA is layout

dependent. For SOP-8 (Exposed Pad) package, the

thermal resistance

θJA is 75°C/W on the standard JEDEC

51-7 four layers thermal test board. For SOP-8 package,

the thermal resistance

θJA is 90°C/W on the standard

JEDEC 51-7 four layers thermal test board. The maximum

power dissipation at TA = 25

°C can be calculated by

following formula :

PD(MAX) = (125

°C − 25°C) / (75°C/W ) = 1.33W

(min. copper area PCB layout with SOP-8 Exposed Pad)

PD(MAX) = (125

°C − 25°C) / (49°C/W ) = 2.04W

(70mm

2 copper area PCB layout with SOP-8 Exposed

Pad)

PD(MAX) = (125

°C − 25°C) / (90°C/W ) = 1.11W

(min. copper area PCB layout with SOP-8)

The thermal resistance

θJA of SOP-8 (Exposed Pad) is

determined by the package architecture design and the

PCB layout design. However, the package architecture

design had been designed. If possible, it's useful to increase

thermal performance by the PCB layout copper design.

The thermal resistance

θJA can be decreased by adding

copper area under the exposed pad of SOP-8 (Exposed

Pad) package.

As shown in Figure 6, the amount of copper area to which

the SOP-8 (Exposed Pad) is mounted affects thermal

performance. When mounted to the standard SOP-8

(Exposed Pad) pad (Figure 6.a),

θJA is 75°C/W. Adding

copper area of pad under the SOP-8 (Exposed Pad) (Figure

6.b) reduces the

θJA to 64°C/W. Even further, increasing

the copper area of pad to 70mm

2 (Figure 6.e) reduces the

θJA to 49°C/W.

The maximum power dissipation depends on operating

ambient temperature for fixed TJ(MAX) and thermal

resistance

θJA. For RT8294 packages, the of de-rating

curves in Figure 7 allow the designer to see the effect of

rising ambient temperature on the maximum power

dissipation allowed.

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