Datasheet

Closing the loop A5972D
14/37 Doc ID 13956 Rev 5
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 8. 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 8. 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
+()•
--------------------------------------------------------=