RT7270
the long wires can potentially cause a voltage spike at
VIN large enough to damage the part.
As shown in Figure 5, 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 5.a), θJA is 75°C/W.
Adding copper area of pad under the SOP-8 (Exposed
Pad) (Figure 5.b) reduces the θJA to 64°C/W. Even further,
increasing the copper area of pad to 70mm2 (Figure 5.e)
reduces the θJA to 49°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 :
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal
resistance θJA. The Figure 6 of derating curves allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation allowed.
PD(MAX) = (TJ(MAX) − TA ) / θJA
Where TJ(MAX) is the maximum operation junction
temperature , TA is the ambient temperature and the θJA is
the junction to ambient thermal resistance.
2.2
Four-Layer PCB
2.0
1.8
For recommended operating conditions specification, 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
Copper Area
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2
70mm
2
50mm
2
30mm
2
10mm
Min.Layout
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)
0
25
50
75
100
125
Ambient Temperature (°C)
PD(MAX) = (125°C − 25°C) / (49°C/W) = 2.04W
(70mm2copper area PCB layout)
Figure 6.Derating Curve of Maximum PowerDissipation
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.
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7270-03 March 2018
www.richtek.com
11
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
