ACT4012A
Rev0, 16-May-08
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 .8
×
10
−
5
R
COMP
(F)
(10)
COMP
ACT4012A
C
COMP
1
C
COMP2
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)
(11)
R
COMP
: C
COMP2
is needed only for high ESR output capacitor
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
ESRCOUT
⎛
1.1
×
10
−
6
⎞
,0.012
×
V
OUT
⎟
(Ω)
≥
Min
⎜
⎜
C
⎟
OUT
⎝
⎠
(12)
A
VDC
=
1 .3V
A
VEA
G
COMP
I
OUT
(4)
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
COMP2
=
(5)
C
OUT
R
ESRCOUT
R
COMP
(13)
Though C
COMP2
is unnecessary when the output ca-
pacitor has sufficiently low ESR, a small value
C
COMP2
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:
(6)
The first zero Z1 is due to R
COMP
and C
COMP
:
f
Z1
=
1
2
π
R
COMP
C
COMP2
(7)
Typical Compensation for Different Output
Voltages and Output Capacitors
V
OUT
2.5V
3.3V
5V
2.5V
3.3V
5V
2.5V
3.3V
5V
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
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
12kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
C
COMP
C
COMP2
1.5nF
1.5nF
1.5nF
1.5nF
1.8nF
2.7nF
15nF
22nF
27nF
None
None
None
None
None
None
1nF
1nF
None
(8)
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
10 G
EA
G
COMP
×
1 .3V
(Ω)
(9)
=
2
×
10
8
V
OUT
C
OUT
: C
COMP2
is needed for high ESR output capacitor.
Figure 3 shows an example ACT4012A application circuit gener-
ating a 5V/1.5A output.
but limit R
COMP
to 15kΩ maximum.
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TM
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