RT7253A datasheet(12/14 Pages) RICHTEK

12 / 14 page

RT7253A

DS7253A-02 March 2011

www.richtek.com

Figure 7. Derating Curves for RT7253A Package

(a) Copper Area = (2.3 x 2.3) mm2,

θJA = 75°C/W

(b) Copper Area = 10mm2,

θJA = 64°C/W

θJA = 54°C/W

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

RT7253A, 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. 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

(70mm2copper 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

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 6a),

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 70mm2 (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 RT7253Apackages, the derating curves

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

ambient temperature on the maximum power dissipation

allowed.

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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