AN1518

Closing the loop

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5

Closing the loop

Figure 9.

Block diagram of the loop

6

Error amplifier and compensation network

The output L-C filter of a step-down converter contributes with 180 degrees phase shift in

the control loop. For this reason a compensation network between the COMP pin and

GROUND is added. The simplest compensation network together with the equivalent circuit

open loop gain. CP does not significantly affect system stability but it is useful to reduce the

noise of the COMP pin.

The transfer function of the error amplifier and its compensation network is:

Equation 2

A

()

A

V0

1s

+

R

c

C

c

•

•

()

•

s

2

R

0

•

C

0

C

p

+

() R

c

C

c

sR

0

C

c

•

R

0

C

0

C

p

+

() R

c

C

c

•

+

•

+

() 1

+

•

+

•

•

•

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=