Active-Semi
Stability compensation
Figure 2:
Stability Compensation
COMP
ACT4072
Rev2, 27-May-08
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
C
COMP
R
COMP
C
COPM2
1 . 8
×
10
−
5
=
R
COMP
(F)
(10)
ACT4072
If R
COMP
is limited to 15kΩ, then the actual cross
over frequency is 3.4/(V
OUT
C
OUT
). Therefore:
C
COMP
=
1.3 x10
−
5
V
OUT
C
OUT
(F)
(11)
: C
COMP2
is needed only for high ESR output capacitors or PCB
parasitics
The feedback system 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
ESROUT
A
VDC
=
1.222V
A
VEA
G
COMP
I
OUT
(4)
⎛
1.1x10
−
6
≥
Min
⎜
,0.012V
OUT
⎜
C
OUT
⎝
⎞
⎟
(Ω)
⎟
⎠
(12)
And the proper value for C
COMP2
is:
The dominant pole P1 is due to C
COMP
:
f
P 1
G
EA
=
2
π
A
VEA
C
COMP
C
COMP2
=
(5)
C
OUT
R
ESROUT
R
COMP
(13)
The second pole P2 is the output pole:
A small value C
COMP2
such as 100pF may improve
stability against PCB layout parasitic effects.
(6)
Table 2 shows some calculated results based on the
compensation method above.
Table 2:
(7)
Typical Compensation for Different Output volt-
ages and Output Capacitors
V
OUT
2.5V
3.3V
f
P 2
=
I
OUT
2
π
V
OUT
C
OUT
1
2
π
R
COMP
C
COMP
The first zero Z1 is due to R
COMP
and C
COMP
:
f
Z 1
=
And finally, the third pole is due to R
COMP
and
C
COMP2
(if C
COMP2
is used):
C
OUT
22µF Ceramic
22µF Ceramic
22µF Ceramic
47µF SP CAP
47µF SP CAP
47µF SP CAP
470µF/6.3V/30mΩ
470µF/6.3V/30mΩ
470µF/6.3V/30mΩ
R
COMP
C
COMP
C
COMP2
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
100pF
100pF
100pF
100pF
100pF
100pF
1nF
1nF
1nF
f
P 3
=
1
2
π
R
COMP
C
COMP2
(8)
5V
2.5V
3.3V
5V
2.5V
3.3V
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
:
R
COMP
2
π
V
OUT
C
OUT
f
SW
=
10G
EA
G
COMP
1.222V
(Ω)
(9)
=
2.3 x10
8
V
OUT
C
OUT
but limit R
COMP
to 15kΩ maximum.
: C
COMP2
is needed for board parasitic and high ESR output
capacitor.
Figure 3 shows an example ACT4072 application
circuit generating a 5V/2A output.
-6-
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