RT7253A
Thermal Considerations
resistance θJA. For RT7253Apackages, the derating curves
in Figure 7 allow the designer to see the effect of rising
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 :
ambient temperature on the maximum power dissipation
allowed.
2.2
Four Layer PCB
2.0
1.8
Copper Area
1.6
2
70mm
2
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
50mm
30mm
10mm
PD(MAX) = (TJ(MAX) − TA ) / θJA
2
2
where TJ(MAX) is the maximum operation junction
temperature , TA is the ambient temperature and the θJA is
the junction to ambient thermal resistance.
Min.Layout
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 :
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 7.Derating Curves for RT7253APackage
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.
(a) Copper Area = (2.3 x 2.3) mm2,θJA = 75°C/W
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.
(b) Copper Area = 10mm2,θJA = 64°C/W
(c) Copper Area = 30mm2 ,θJA = 54°C/W
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal
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DS7253A-02 March 2011
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
