RT9206
Application Information
Synchronous Buck Converter
is1
is2
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
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 (DIL) between 20% and 50% of output current is
appropriate. Figure 1 shows the typical topology of
synchronous step-down converter and its related
waveforms.
Figure 1.The waveforms of synchronous step-down
converter
According to Figure 1 the ripple current of inductor can be
calculated as follows :
D
VOUT
VIN
VIN - VOUT = L ΔIL ;Δt = ;D =
Δt
fs
VOUT
(1)
L = (VIN - VOUT)´
VIN ´ fs ´ ΔIL
i
i
L
L
S1
Where :
VIN= Maximum input voltage
= Output Voltage
+ V -
L
I
i
OUT
C
i
S2
+
S1
+
r
V
V
OUT
C
OR
-
R
S2
L
V
V
OUT
IN
Dt=S1 turn on time
DIL=Inductor current ripple
fS= Switching frequency
D=Duty Cycle
+
V
C
OC
-
OUT
-
Ts
rC= Equivalent series resistor of output capacitor
Vg1
Vg2
Ton
Toff
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.
VIN-VOUT
VL
-VOUT
iL
IL=IOUT
ΔIL
DS9206-11 March 2007
www.richtek.com
13
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
