RT7248
increasing the copper area of pad to 70mm2 (Figure 7.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. The Figure 8 of derating curves allows 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
Copper Area
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2
70mm
2
50mm
30mm
2
2
10mm
Min.Layout
(b) Copper Area = 10mm2,θJA = 64°C/W
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 8. Derating Curve of Maximum PowerDissipation
(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. CopperArea Layout
Design
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7248-01 September 2012
www.richtek.com
13
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