RT8280 データシートの表示（PDF） - Richtek Technology

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RT8280

3A, 24V, 3MHz Step-Down Converter

Richtek Technology 

RT8280

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX) − TA) / θJA

where TJ(MAX) is the maximum junction temperature, TA is

the ambient temperature, and θJA is the junction to ambient

thermal resistance.

For recommended operating condition specifications, the

maximum junction temperature is 125°C. The junction to

ambient thermal resistance, θJA, is layout dependent. For

SOP-8 (Exposed Pad) packages, the thermal resistance,

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

thermal test board. The maximum power dissipation at

TA=25°C can be calculated by the following formulas :

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

(70mm2 copper area PCB layout)

The thermal resistance, θJA, of SOP-8 (Exposed Pad) is

determined by the package architectural design and the

PCB layout design. The package architectural design is

fixed. However, it's possible to increase thermal

performance via better PCB layout copper design. The

thermal resistance, θJA, can be decreased by adding

copper area under the exposed pad of the SOP-8

(Exposed Pad) package.

As shown in Figure 7, the amount of copper area to which

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

performance. When mounted to the standard SOP-8

(Exposed Pad) (Figure 7a), θJA is 75°C/W. Adding copper

area under the SOP-8 (Exposed Pad) (Figure 7b) reduces

θJA to 64°C/W. Further increasing the copper area to

70mm2 (Figure 7e) will reduce θJA to 49°C/W.

The maximum power dissipation depends on operating

ambient temperature for fixed TJ (MAX) and thermal

resistance, θJA. The derating curves in Figure 8 allow the

Copyright ©2012 Richtek Technology Corporation. All rights reserved.

DS8280-02 March 2012

designer to see the effect of rising ambient temperature

on the maximum power dissipation.

(a) Copper Area = (2.3 x 2.3) mm2, θJA = 75°C/W

(b) Copper Area = 10mm2, θJA = 64°C/W

(c) Copper Area = 30mm2 , θJA = 54°C/W

(d) Copper Area = 50mm2 , θJA = 51°C/W

(e) Copper Area = 70mm2 , θJA = 49°C/W

Figure 7. Themal Resistance vs. Copper Area Layout

Design

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