RT8256 데이터 시트보기 (PDF) - Richtek Technology
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RT8256 Datasheet PDF : 12 Pages
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RT8256
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to ΔILOAD (ESR) and also begins to charge or
discharge COUT generating a feedback error signal for the
regulator to return VOUT to its steady-state value. During
this recovery time, VOUT can be monitored for overshoot or
ringing that would indicate a stability problem.
Thermal Considerations
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 :
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.
For recommended operating conditions specification of
RT8256, the maximum junction temperature is 125°C. The
junction to ambient thermal resistance θJA for SOP-8
package is 120°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) / (120°C/W) = 0.833W for
SOP-8 packages
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal
resistance θJA. For RT8256 packages, the Figure 3 of
derating curves allows the designer to see the effect of
rising ambient temperature on the maximum power
allowed.
1.0
0.9
Four Layer PCB
0.8
0.7
0.6
SOP-8
0.5
0.4
0.3
0.2
0.1
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 3. Derating Curves for RT8256 Packages
Layout Consideration
Follow the PCB layout guidelines for optimal performance
of the RT8256.
` Keep the traces of the main current paths as short and
wide as possible.
` Put the input capacitor as close as possible to the device
pins (VIN and GND).
` LX node is with high frequency voltage swing and should
be kept at small area. Keep sensitive components away
from the LX node to prevent stray capacitive noise pick-
up.
` Place the feedback components to the FB pin as close
as possible.
` The GND should be connected to a strong ground plane
for heat sinking and noise protection.
www.richtek.com
10
DS8256-01 March 2011
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