Active-Semi
STABILITY COMPENSATION
Figure 2:
Stability Compensation
ACT4050
Rev 2, 01-Jul-11
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
COMP
1.6
×
10
−
5
=
R
COMP
(F)
(13)
ACT4050
C
COMP
C
COMP2
R
COMP
If R
COMP
is limited to 15kΩ, then the actual cross
over frequency is 3.4 / (V
OUT
C
OUT
). Therefore:
C
COMP
=
1.2
×
10
−
5
V
OUT
C
OUT
(F)
(14)
: C
COMP2
is needed only for high ESR output capacitor
The feedback loop of the IC is stabilized by the
components at the 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 using
C
COMP2
is:
R
ESRCOUT
⎛
1 .1
×
10
−
6
≥
Min
⎜
,0 .012
×
V
OUT
⎜
C
OUT
⎝
⎞
⎟
(Ω) (15)
⎟
⎠
A
VDC
=
0 . 82 V
A
VEA
G
COMP
I
OUT
(7)
And the proper value for C
COMP2
is:
The dominant pole P1 is due to C
COMP
:
G
EA
f
P1
=
2
π
A
VEA
C
COMP
The second pole P2 is the output pole:
f
P 2
=
I
OUT
2
π
V
OUT
C
OUT
C
COMP 2
=
(8)
C
OUT
R
ESRCOUT
R
COMP
(16)
Though C
COMP2
is unnecessary when the output
capacitor has sufficiently low ESR, a small value
C
COMP2
such as 100pF may improve stability against
PCB layout parasitic effects.
Table 3 shows some calculated results based on
the compensation method above.
Table 2:
(9)
The first zero Z1 is due to R
COMP
and C
COMP
:
f
Z1
=
1
2
π
R
COMP
C
COMP
(10)
Typical Compensation for Different Output
Voltages and Output Capacitors
V
OUT
C
OUT
R
COMP
C
COMP
C
COMP2
And finally, the third pole is due to R
COMP
and
C
COMP2
(if C
COMP2
is used):
f
P 3
=
1
2
π
R
COMP
C
COMP2
2.5V
3.3V
5V
2.5V
3.3V
5V
2.5V
3.3V
5V
2x22μF Ceramic
2x22μF Ceramic
2x22μF Ceramic
47μF SP CAP
47μF SP CAP
47μF SP CAP
470μF/6.3V/30mΩ
470μF/6.3V/30mΩ
470μF/6.3V/30mΩ
8.2kΩ
12kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
2.2nF
1.5nF
1.5nF
1.5nF
1.8nF
2.7nF
15nF
22nF
27nF
None
None
None
None
None
None
1nF
1nF
None
(11)
The following steps should be used to compensate
the IC:
STEP 1. Set the cross over frequency at 1/10 of the
switching frequency via R
COMP
:
R
COMP
=
2
π
V
OUT
C
OUT
f
SW
10 G
EA
G
COMP
×
0 .82 V
8
: C
COMP2
is needed for high ESR output capacitor.
=
1 . 88
×
10 V
OUT
C
OUT
(Ω)
(12)
Figure 3 shows an example ACT4050 application circuit
generating a 2.5V/3.5A output.
but limit R
COMP
to 15kΩ maximum.
Innovative Power
TM
-7-
www.active-semi.com
Copyright © 2011 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
