RT8293B datasheet(12/14 Pages) RICHTEK

12 / 14 page

RT8293B

DS8293B-03

March 2011

www.richtek.com

Thermal Considerations

For continuous operation, do not exceed the 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 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 , TA is the ambient temperature and the

θJAis

the junction to ambient thermal resistance.

For recommended operating conditions specification of

RT8293B, the maximum junction temperature is 125

°C.

The junction to ambient thermal resistance

θJA is layout

dependent. For PSOP-8 package, the thermal resistance

θJA is 75°C/W on the standard JEDEC 51-7 four-layer

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) = 1.333W

(min.copper area PCB layout)

PD(MAX) = (125

°C − 25°C) / (49°C/W ) = 2.04W

(70mm

2copper area PCB layout)

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

θJAcan 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

per form ance. W hen m ount ed to the standard

SOP-8 (Exposed Pad) pad (Figure 6a.),

θJA is 75°C/W.

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

Pad) (Figure 6b.) reduces the

θJA to 64°C/W. Even further,

increasing the copper area of pad to 70mm

2 (Figure 6e.)

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 RT8293B package, the Figure 7. of

derating curves allows the designer to see the effect of

rising ambient temperature on the maximum power

dissipation allowed.

Figure 7. Derating Curves for RT8293B Package

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

θJA = 75°C/W

(b) Copper Area = 10mm

θJA = 64°C/W

2 ,

θJA = 54°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

Four-Layer PCB

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