ADP1822

Rev. B | Page 17 of 24

or

In all three cases, it is sometimes beneficial, although not required,

to FB to reduce high frequency noise. This capacitor forms an

extra pole in the loop response. Choose this capacitor such that

the pole occurs at approximately 1/2 of the switching frequency, or

()(

)

COMP

C

COMP

SW

PC

R

C

C

f

F

2

2

//

π

2

1

2

=

=

(22)

()(

)

COMP

SW

C

R

f

C

π

2

2

(23)

Compensation Using the ESR Zero

TO

PWM

COMP

FB

0.6V

INTERNAL ERROR AMPLIFIER

Figure 18. Compensation Using the ESR Zero

If the output capacitor ESR zero is sufficiently low (less than or

equal to 1/2 of the crossover frequency), use the ESR to stabilize

the regulator. In this case, use the circuit shown in Figure 16.

Choose the compensation resistor to set the desired crossover

frequency, typically 1/10 of the switching frequency or

()

(

()

)

2

LC

IN

CO

ESRZ

RAMP

TOP

COMP

f

V

f

f

V

R

R

=

(24)

where:

LC

f

LC

π

2

1

=

(25)

using known constants

()(

)( )( )( )

IN

SW

ESRZ

TOP

COMP

V

C

L

f

f

R

R

9

.

4

≅

(26)

Choose the compensation capacitor to set the compensation

the LC resonant frequency, or

()(

)

COMP

COMP

SW

CO

ZC

R

C

f

f

f

π

2

1

40

4

=

=

=

(27)

()(

)

COMP

COMP

LC

ZC

R

C

f

f

π

2

1

2

=

=

(28)

()(

)

COMP

CO

COMP

R

f

C

π

2

4

=

(29)

In terms of the switching frequency and combining the constants,

()(

)

COMP

SW

COMP

R

f

C

37

.

6

≅

(30)

or

()(

)

COMP

LC

COMP

R

f

C

π

2

2

=

(31)

or whichever is greater.

Compensation Using Feed-Forward

TO

PWM

COMP

FB

0.6V

INTERNAL ERROR AMPLIFIER

Figure 19. Compensation Using Feed-Forward

If the ESR zero is at too high a frequency to be useful in

stabilizing the regulator, add a series RC network, as shown in

Figure 17, in parallel with the top side voltage divider resistor,

increase the phase at crossover, thus improving stability.

Choose the feed-forward zero frequency for 1/7 of the crossover

frequency, and the feed-forward pole at 7× the crossover

frequency. This sets the ratio of pole-to-zero frequency of

approximately 50:1 for optimum stability.

frequency by

(

)(

)(

)( )

LC

IN

CO

ZFF

RAMP

TOP

COMP

f

V

f

f

V

R

R

=

(32)

crossover frequency. Simplify the following equation:

(

IN

SW

TOP

COMP

V

C

L

f

R

R

2

0705

.

0

≅

(33)