6

 2003 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2612A/C

COMPENSA

COMPENSA

COMPENSA

COMPENSA

COMPENSATION COMPONENTS

TION COMPONENTS

TION COMPONENTS

TION COMPONENTS

TION COMPONENTS - Once the filter com-

ponents have been determined, the compensation com-

ponents can be calculated. The goal of compensation is

to modify the frequency response characteristics of the

error amplifier to ensure that the closed loop feedback

system has the highest gain and bandwidth possible while

maintaining stability.

A simplified stability criteria states that the open loop

gain of the converter should fall through 0dB at 20dB/

decade at a frequency no higher than 20-25% of the

switching frequency.

This objective is most simply met by generating asymp-

totic bode plots of the small signal response of the vari-

ous sections of the converter.

L

VOUT

Co

SC2612 AND FETS

FB

OUT

COMP

Ra

MODULATOR

REF

+

-

EA

Rb

Resr

Zp

Zf

Zs

It is convenient to split the converter into two sections,

the Error amp and compensation components being one

section and the Modulator, output filter and divider be-

ing the other.

First calculate the DC Filter+Modulator+Divider gain

The DC filter gain is always 0dB, the Modulator gain is

19dB at 5V in and is proportional to Vin, so modulator

gain at any input voltage is.

÷

ø

ö

ç

è

æ

×

+

=

5

20

19

IN

MOD

V

Log

G

the divider gain is given by

÷÷ø

ö

ççè

æ

+

×

=

8

5

8

20

R

R

R

Log

G

DIV

So the total Filter+Modulator+Divider DC Gain is

÷÷ø

ö

ççè

æ

+

×

+

÷

ø

ö

ç

è

æ

×

+

=

B

A

B

IN

FMD

R

R

R

Log

V

Log

G

20

5

20

19

Calculate the filter double pole frequency (Fp(lc))

LCo

)

lc

(

Fp

p

=

2

1

and calculate ESR Zero frequency (Fz(esr))

sr

Re

Co

)

esr

(

Fz

×

×

p

=

2

1

Choose an open loop crossover frequency (Fco) no higher

than 20% of the switching frequency (Fs).

The proximity of Fz(esr) to the crossover frequency Fco

determines the type of compensation required, if

Fz(esr)>Fco/4, use type 3 compensation, otherwise use

type 2. Type 1 compensation is not appropriate and is

not discussed here.

Type 2 Example

As an example of type 2 compensation, we will use the

Evaluation board schematic.

3.3uH

VOUT

3000uF

SC2612 AND FETS

FB

OUT

COMP

6.98k

MODULATOR

REF

+

-

EA

8.06k

22mOhm

Cs

Cp

Rs

Vin=5V

The total Filter+Modulator+Divider DC Gain is

dB

.

.

.

.

Log

Log

G

FMD

6

13

06

8

98

6

06

8

20

5

5

20

19

=

÷

ø

ö

ç

è

æ

+

×

+

÷

ø

ö

ç

è

æ

×

+

=

This is drawn as the line A-B in Fig2

kHz

.

.

LCo

)

lc

(

Fp

6

1

10

3000

10

3

3

2

1

2

1

6

6

»

×

×

×

p

=

p

=

-

-

This is point B in Fig2.

kHz

.

)

esr

(

Fz

4

2

10

22

10

3000

2

1

3

6

=

×

×

×

×

p

=

-

-

This is point C in Fig2., the line joining B-C slopes at -

40dB/decade, the line joining C-D slopes at -20dB/de-

cade.

For 600kHz switching frequency, crossover is designed

for 100kHz.

Since Fz(esr)<<Fco/4 Type 2 compensation is appropri-

ate.