APW7065C 데이터 시트보기 (PDF) - Anpec Electronics

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APW7065C

Synchronous Buck PWM Controller

Anpec Electronics 

APW7065C

Application Information (Cont.)

PWM Compensation (Cont.)

The poles and zeros of the transfer function are:

FZ1

=

1

2 × π × R2 × C2

FZ2

=

2×

1

π × (R1+

R3)× C3

FP1

=

1

2 × π × R2 × 

C1× C2



 C1+ C2 

FP2

=

1

2 × π × R3 × C3

C1

R3 C3

R2 C2

VOUT

R1 FB

VCOMP

3.Place the first zero FZ1 before the output LC filter double

pole frequency FLC.

FZ1 = 0.75 X FLC

Calculate the C2 by the equation:

C2 =

1

2 × π × R2 × FLC × 0.75

4.Set the pole at the ESR zero frequency FESR:

FP1 = FESR

Calculate the C1 by the equation:

C1 =

C2

2 × π × R2 × C2 × FESR − 1

5.Set the second pole FP2 at the half of the switching fre-

quency and also set the second zero FZ2 at the output

LC filter double pole F . The compensation gain should

LC

not exceed the error amplifier open loop gain, check the

compensation gain at FP2 with the capabilities of the error

amplifier.

VREF

Figure 6. Compensation Network

The closed loop gain of the converter can be written as:

GAIN X GAIN X GAIN

LC

PWM

AMP

Figure 7. shows the asymptotic plot of the closed loop

converter gain, and the following guidelines will help to

design the compensation network. Using the below

guidelines should give a compensation similar to the

curve plotted. A stable closed loop has a -20dB/ decade

slope and a phase margin greater than 45 degree.

FP2 = 0.5 X FS

F =F

Z2

LC

Combine the two equations will get the following com-

ponent calculations:

R3 =

R1

FS − 1

2 × FLC

C3 =

1

π × R3 × FS

1.Choose a value for R1, usually between 1K and 5K.

2.Select the desired zero crossover frequency F :

O

(1/5 ~ 1/10) X FS >FO>FESR

Use the following equation to calculate R2:

R2 = ∆VOSC × FO × R1

VIN FLC

Copyright © ANPEC Electronics Corp.

14

Rev. A.2 - Mar., 2008

www.anpec.com.tw

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