RT9206GS datasheet(13/21 Pages) RICHTEK

13 / 21 page

RT9206

DS9206-11

March 2007

www.richtek.com

Application Information

Synchronous Buck Converter

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 adv antages, 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 (

∆IL) between 20% and 50% of output current is

appropriate. Figure 1 shows the typical topology of

synchronous step-down conv erter 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 :

(1)

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.

Vg1

Vg2

Ton

Toff

VIN-VOUT

-VOUT

IL=IOUT

ΔIL

is1

is2

××

OUT

INL

ΔI

V - V

= L;

Δt =

;D=

Δt

fsV

L = (V - V)

ΔI

+ V

L -

OUT

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