ACT4070
Stability compensation
Figure 2:
Stability Compensation
STEP 2. Set the zero f
Z1
at 1/4 of the cross over
frequency. If R
COMP
is less than 15kΩ, the equation
for C
COMP
is:
C
COMP
1.6 x10
5
�½
R
COMP
(F)
(10)
If R
COMP
is limited to 15kΩ, then the actual cross
over frequency is 4.8/(V
OUT
C
OUT
). Therefore:
C
COMP
�½
8.8 x10
6
V
OUT
C
OUT
:
C
COMP2
is needed only for high ESR output capacitor
(F)
(11)
The feedback system of the IC is stabilized by the
components at COMP pin, as shown in Figure 2.
The DC loop gain of the system is determined by
the following equation:
STEP 3. If the output capacitor’s ESR is high
enough to cause a zero at lower than 4 times the
cross over frequency, an additional compensation
capacitor C
COMP2
is required. The condition for us-
ing C
COMP2
is:
R
ESROUT
1.1x10
6
Min
,0.012V
OUT
C
OUT
And the proper value for C
COMP2
is:
(Ω)
(12)
A
VDC
1.222V
�½
A
VEA
G
COMP
I
OUT
(4)
The dominant pole P1 is due to C
COMP
:
f
P 1
�½
G
EA
2
A
VEA
C
COMP
(5)
C
COMP
�½
C
OUT
R
ESROUT
R
COMP
(13)
The second pole P2 is the output pole:
f
P 2
�½
I
OUT
2
V
OUT
C
OUT
(6)
Though C
COMP2
is unnecessary when the output
capacitor has sufficiently low ESR, a small value
C
COMP2
such as 220pF may improve stability
against PCB layout parasitic effects.
Table 2 shows some calculated results based on
the compensation method above.
The first zero Z1 is due to R
COMP
and C
COMP
:
f
Z1
�½
1
2
R
COMP
C
COMP
(7)
Table 2:
Typical Compensation for Different Output
Voltages and Output Capacitors
V
OUT
C
OUT
22μF Ceramic
22μF Ceramic
22μF Ceramic
100μF SP CAP
100μF SP CAP
100μF SP CAP
And finally, the third pole is due to R
COMP
and
C
COMP2
(if C
COMP2
is used):
f
P 3
�½
1
2
R
COMP
C
COMP2
R
COMP
4kΩ
5.6kΩ
12kΩ
15kΩ
15kΩ
15kΩ
C
COMP
C
COMP2
3.3nF
3.3nF
1.5nF
1.5nF
2.2nF
4.7nF
220pF
220pF
220pF
220pF
220pF
220pF
(8)
1.8V
2.5V
Follow the following steps to compensate the IC:
STEP 1. Set the cross over frequency at 1/10 of
the switching frequency via R
COMP
:
5V
1.8V
2.5V
R
COMP
�½
2
V
OUT
C
OUT
f
SW
10 G
EA
G
COMP
1 .222 V
8
5V
�½
1 .25 x10 V
OUT
C
OUT
(Ω)
(9)
:
C
COMP2
is needed for board parasitic and high ESR output
capacitor.
but limit R
COMP
to 15kΩ maximum.
Innovative Products.
Active Solutions.
-6-
Figure 3 shows a sample ACT4070 application
circuit generating a 2.5V/3A output.
Copyright © 2006 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
