ACT4012
STABILITY COMPENSATION
*CCOMP2 is needed only for high ESR output capacitor
Figure 4. Stability Compensation
The feedback system of the IC is stabilized
by the components at COMP pin, as shown in
Figure 4. The DC loop gain of the system is
determined by the following equation:
COMP
VEA
OUT
VDC
G
A
I
V
3
.
1
A
=
(4)
The dominant pole P1 is due to CCOMP:
COMP
VEA
EA
1
P
C
A
π
2
G
f
=
(5)
The second pole P2 is the output pole:
OUT
OUT
OUT
2
P
C
V
π
2
I
f
=
(6)
The first zero Z1 is due to RCOMP and CCOMP:
COMP
COMP
1
Z
C
R
π
2
1
f
=
(7)
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
2
COMP
COMP
3
P
C
R
π
2
1
f
=
(8)
Follow the following steps to compensate the
IC:
STEP 1. Set the cross over frequency at 1/10 of
the switching frequency via RCOMP:
V
3
.
1
G
G
10
f
C
V
π
2
R
COMP
EA
SW
OUT
OUT
COMP
•
=
)
(
C
V
10
7
.
1
OUT
OUT
8
Ω
×
=
(9)
but limit RCOMP to 15kΩ maximum.
STEP 2. Set the zero fZ1 at 1/4 of the cross over
frequency. If RCOMP is less than 15kΩ, the
equation for CCOMP is:
)
F
(
R
10
8
.
1
C
COMP
5
COMP
−
×
=
(10)
If RCOMP is limited to 15kΩ, then the actual cross
over frequency is 3.4 / (VOUTCOUT). Therefore:
)
F
(
C
V
10
2
.
1
C
OUT
OUT
5
COMP
−
×
=
(11)
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 CCOMP2 is required. The
condition for using CCOMP2 is:
ESRCOUT
R
)
(
V
012
.
0
,
C
10
1
.
1
Min
OUT
OUT
6
Ω
•
×
≥
−
(12)
And the proper value for CCOMP2 is:
COMP
ESRCOUT
OUT
2
COMP
R
R
C
C
=
(13)
Though CCOMP2 is unnecessary when the
output capacitor has sufficiently low ESR, a
small value CCOMP2 such as 100pF may improve
stability against PCB layout parasitic effects.
Table 2 shows some calculated results based
on the compensation method above.
Table 2. Typical Compensation for Different
Output Voltages and Output Capacitors
VOUT
COUT
RCOMP
CCOMP
CCOMP2
2.5V
22μF Ceramic
8.2kΩ
2.2nF
None
3.3V
22μF Ceramic
12kΩ
1.5nF
None
5V
22μF Ceramic
15kΩ
1.5nF
None
2.5V
47μF SP Cap
15kΩ
1.5nF
None
3.3V
47μF SP Cap
15kΩ
1.8nF
None
5V
47μF SP Cap
15kΩ
2.7nF
None
2.5V
470μF/6.3V/30mΩ 15kΩ
15nF
1nF
3.3V
470μF/6.3V/30mΩ 15kΩ
22nF
1nF
5V
470μF/10V/30mΩ
15kΩ
27nF
None
Figure
5
shows
a
sample
ACT4012
application circuit generating 2.5V/2A output.
Active-Semi, Inc.
- 6 -
www.active-semi.com
ACT4012
COMP
C
C OMP
R
C OMP
CCOMP2
*
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