RT9218B Просмотр технического описания (PDF) - Richtek Technology
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RT9218B Datasheet PDF : 17 Pages
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RT9218B
Application Information
Inductor Selection
The selection of output inductor is based on the
considerations of efficiency, output power and operating
frequency. Low inductance value has smaller size, but
results in low efficiency, large ripple current and high output
ripple voltage. Generally, an inductor that limits the ripple
current (ΔIL) between 20% and 50% of output current is
appropriate. Figure 1 shows the typical topology of
synchronous step-down converter and its related
waveforms.
iS1
S1
VIN
S2
L
IL
+ VL -
iS2
+
VOR
-
+
VOC
-
iC
rC
RL
COUT
IOUT
+
VOUT
-
TS
Vg1 TON TOFF
Vg2
VL
VIN - VOUT
- VOUT
iL
ΔIL
IL = IOUT
According to Figure 1 the ripple current of inductor can be
calculated as follows :
VIN
− VOUT
= L ΔIL
Δt
;
Δt = D ;
fs
D = VOUT
VIN
L
=
(VIN
−
VOUT
)×
VIN
VOUT
× fs ×
ΔIL
(1)
Where :
VIN = Maximum input voltage
VOUT = Output Voltage
Δt = S1 turn on time
ΔIL = Inductor current ripple
fS = Switching frequency
D = Duty Cycle
rC = Equivalent series resistor of output capacitor
Output Capacitor
The selection of output capacitor depends on the output
ripple voltage requirement. Practically, the output ripple
voltage is a function of both capacitance value and the
equivalent series resistance (ESR) rC. Figure 2 shows the
related waveforms of output capacitor.
iL
dditL=
VIN-VOUT
L
diL
dt
=
VOUT
L
IOUT
TS
iC
1/2ΔIL
0
ΔIL
iS1
iS2
Figure 1. The waveforms of synchronous step-down
converter
DS9218B-09 March 2007
VOC
VOR
0
ΔVOC
ΔIL x rc
t1
t2
Figure 2. The related waveforms of output capacitor
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