RT9206 Ver la hoja de datos (PDF) - Richtek Technology

Número de pieza

componentes Descripción

Fabricante

RT9206

High Efficiency, Synchronous Buck with Dual Linear Controllers

Richtek Technology 

RT9206

iS1

VIN

S1

S2

d

-

+

PWM

Generator

L

iL

+ VL -

iS2

+

VOR

-

+

VOC

-

iC

rC

RL

COUT

IOUT

+

VOUT

-

Compensator

VREF

Sensor

Gain

Ra

VC

Cc2

gm+

-

Rc1

Cc1

Rb

Next, deriving the transfer function d(s)/vC (s) of the direct

duty ratio pulse-width modulator (PWM Generator). The

transfer function Tm(s) of the modulator is given by

Tm(S) = d(S) = 1

VC(S) Vr

(19)

where, Vr is the amplitude of ramp-waveform which is listed

in datasheet.

For simplification, the transfer function of PWM generator

and Buck converter can is combined. The resulting is shown

in equation (20)

G(S)

=

VOUT(S)

VC(S)

=

S2

xL

1+ rc x CO x S

+ CO x S( L + rc

x

x

CO) + 1

V IN

Vr

RL

(20)

The transfer function of Equation (20) is a second order

system and Bode plot is shown in Figure 7.

Figure 5. The simplified diagram for synchronous Buck

converter and control loop.

Gain

VIN/Vr

From control system point of view, the block diagram of

Figure 5 is shown in Figure 6.

VREF +

-

Compensator

C(s) Vc(s)

G(S)

PWM

Generator

BUCK

Converter

1/Vr d(s) Gp(s)

VOUT

Rb H (s)

Ra+Rb

Sensor Gain

Figure 6. The control block diagram of synchronous

Buck converter

f

fp

fz

Phase

0°

f

-90°

-180°

Figure 7. The Bode plot of Buck power stage

First, deriving the accurate small-signal models of power

stage, the equation (18) is the transfer function of

vO(s)/d(s), which be obtained by space averaging

technique.

GP(S)

=

VOUT(S)

d(S)

=

S2

x

L

1+ rc x CO x S

x CO + S( L + rc

x

CO)

+1

x

VIN

RL

(18)

In Figure 7, the resonance of the output LC filter produces

a double pole and −40dB/decade slop. The resonance

frequency is expressed as follows :

fP =

1

2π x L x CO

(Hz) (21)

DS9206-11 March 2007

www.richtek.com

17

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