Closing the loop

A5974AD

20/49

Doc ID 018762 Rev 1

7.1

Error amplifier and compensation network

The output LC filter of a step-down converter contributes with 180-degree 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 of the

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

Figure 10.

Error amplifier equivalent circuit and compensation network

Equation 3

Equation 4

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

+

⋅

+

⋅⋅

⋅

⋅

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

=

F

P1

1

2

π R

0

C

c

⋅⋅

⋅

----------------------------------

=

F

P2

1

2

π R

c

C

0

C

p

+

()

⋅⋅

⋅

----------------------------------------------------

=