Datasheet

A5973D Closing the loop
17/41
7.1 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
of the error amplifier are shown in Figure 10. R
C
and C
C
introduce a pole and a zero in the
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
Where A
vo
= G
m
· R
o
Figure 10. Error amplifier equivalent circuit and compensation network
The poles of this transfer function are (if C
c
>> C
0
+C
P
):
Equation 3
Equation 4
A
0
s()
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
+()•
--------------------------------------------------------=