RT4531
Thermal Considerations
Layout Consideration
For continuous operation, do not exceed absolute
maximum operation junction temperature. 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 :
For best performance of the RT4531, the following
guidelines must be strictly followed.
Input and Output capacitors should be placed close to
the IC and connected to ground plane to reduce noise
coupling.
The GND and Exposed Pad should be connected to a
strong ground plane for heat sinking and noise protection.
PD(MAX) = ( TJ(MAX) − TA ) / θJA
Keep the main current traces as possible as short and
Where TJ(MAX) is the maximum operation junction
temperature, TA is the ambient temperature and the θJAis
the junction to ambient thermal resistance.
wide.
SW node of DC/DC converter is with high frequency
voltage swing. It should be kept at a small area.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
WL-CSP-9B 1.35x1.35 (BSC) package, the thermal
resistance, θJA, is 81.5°C/W on the standard JEDEC 51-
7 four-layer thermal test board. The maximum power
dissipation at TA = 25°C can be calculated by following
formula :
It recommended to separate the ISET line and PWM
line that avoid the PWM signal couple to ISET.
PD(MAX) = (125°C − 25°C) / (81.5°C/W) = 1.22W for
WL-CSP-9B 1.35x1.35 (BSC) package
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal
resistance θJA. The derating curve in Figure 3 of derating
curves allows the designer to see the effect of rising
ambient temperature on the maximum power allowed.
1.6
Four-Layer PCB
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 3.Derating Curve of Maximum PowerDissipation
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is a registered trademark of Richtek Technology Corporation.
DS4531-01 July 2016
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