RT9206 Ver la hoja de datos (PDF) - Richtek Technology
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RT9206 Datasheet PDF : 21 Pages
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RT9206
Application Information
Synchronous Buck Converter
is1
The RT9206 is specifically designed for synchronous buck
converter with wide input voltage from 4.75V to 28V and
operating frequency from 200kHz to 600kHz. To fully utilize
its advantages, peripheral components should be
is2
appropriately selected. The following information provides
basic considerations for component selection.
Output 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 (∆IL) 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
+ VL -
iS2
iL
+
VOR
-
+
VOC
-
iC
rC
RL
COUT
IOUT
+
VOUT
-
Ts
Vg1
Ton Toff
Vg2
VL
iL
ΔIL
V IN -V OU T
-VOUT
IL=IOUT
Figure 1.The waveforms of synchronous step-down
converter
According to Figure 1 the ripple current of inductor can be
calculated as follows :
VIN - VOUT = L ΔIL ; Δt = D ;D = VOUT
Δt
fs
VIN
L = (VIN - VOUT) ×
VOUT
(1)
VIN × fs × ΔIL
Where :
VIN= Maximum input voltage
VOUT= Output Voltage
∆t=S1 turn on time
∆IL=Inductor current ripple
fS= Switching frequency
D=Duty Cycle
rC= Equivalent series resistor of output capacitor
Output Capacitor Selection
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.
DS9206-11 March 2007
www.richtek.com
13
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