RT8251
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 RT8251 packages, the derating curves
in Figure 7 and Figure 8 allow the designer to see the
effect of rising ambient temperature on the maximum power
dissipation allowed.
(a) Copper Area = (2.3 x 2.3) mm2,θJA = 75°C/W
2.2
Four Layer PCB
2.0
1.8
(b) Copper Area = 10mm2,θJA = 64°C/W
Copper Area
1.6
2
70mm
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2
2
2
50mm
30mm
10mm
Min.Layout
(c) Copper Area = 30mm2 ,θJA = 54°C/W
0
25
50
75
100
125
(°C)
Ambient Temperature
Figure 7. Derating Curves for PSOP-8 Package
1.6
Four Layer PCB
1.4
1.2
(d) Copper Area = 50mm2 ,θJA = 51°C/W
1.0
WQFN-16L 3x3
0.8
0.6
0.4
0.2
0.0
0
15
30
45
60
75
90 105 120 135
(e) Copper Area = 70mm2 ,θJA = 49°C/W
Ambient Temperature (°C)
Figure 8.Derating Curves for WQFNPackage
Figure 6. Themal Resistance vs. CopperArea Layout
Design
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is a registered trademark of Richtek Technology Corporation.
www.richtek.com
12
DS8251-04 February 2013
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
