ACT4065
STABILITY COMPENSATION
COMP
frequency. If R
COMP
is less than 15kΩ, the
equation for C
COMP
is:
ACT4065
C
C O MP
C
C O MP 2
*
C
COMP
1.6
×
10
−
5
=
R
COMP
(F )
(10)
R
C O MP
If R
COMP
is limited to 15kΩ, then the actual cross
over frequency is 6.1 / (V
OUT
C
OUT
). Therefore:
C
COMP
=
6.96
×
10
−
6
V
OUT
C
OUT
*C
COMP2
is needed only for high ESR output capacitor
(F )
(11)
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:
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
A
VDC
=
1.293V
A
VEA
G
COMP
I
OUT
G
EA
(4)
1.1
×
10
−
6
≥
Min
,0.012
•
V
OUT
C
OUT
C
OUT
R
ESRCOUT
R
COMP
(Ω)
(12)
The dominant pole P1 is due to C
COMP
:
f
P1
=
2 πA
VEA
C
COMP
And the proper value for C
COMP2
is:
(5)
C
COMP 2
=
(13)
The second pole P2 is the output pole:
f
P 2
=
I
OUT
2πV
OUT
C
OUT
(6)
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 2 shows some calculated results based
on the compensation method above.
The first zero Z1 is due to R
COMP
and C
COMP
:
f
Z 1
=
1
2πR
COMP
C
COMP
(7)
Table 2. Typical Compensation for Different
Output Voltages and Output Capacitors
And finally, the third pole is due to R
COMP
and
C
COMP2
(if C
COMP2
is used):
f
P 3
=
1
2πR
COMP
C
COMP 2
(8)
IC:
Follow the following steps to compensate the
STEP 1. Set the cross over frequency at 1/5 of
the switching frequency via R
COMP
:
R
COMP
=
2πV
OUT
C
OUT
f
SW
10G
EA
G
COMP
•
1.293V
V
OUT
2.5V
3.3V
5V
2.5V
3.3V
5V
2.5V
3.3V
5V
C
OUT
R
COMP
22μF Ceramic
5.6kΩ
22μF Ceramic
7.2kΩ
22μF Ceramic
10kΩ
47μF SP Cap
11kΩ
47μF SP Cap
15kΩ
47μF SP Cap
15kΩ
470μF/6.3V/30mΩ 15kΩ
470μF/6.3V/30mΩ 15kΩ
470μF/10V/30mΩ 15kΩ
C
COMP
2.7nF
2.2nF
1.5nF
1.5nF
1nF
1.5nF
8.2nF
10nF
15nF
C
COMP2
None
None
None
None
None
None
1nF
1nF
None
=
9.8
×
10
7
V
OUT
C
OUT
(
Ω
)
(9)
but limit R
COMP
to 15kΩ maximum.
STEP 2. Set the zero f
Z1
at 1/4 of the cross over
Figure 5 shows a sample ACT4065
application circuit generating 2.5V/2A output.
Active-Semi, Inc.
-6-
www.active-semi.com
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
