Active-Semi
STABILITY COMPENSATION
Figure 4:
Stability Compensation
ACT4514
Rev 1, 21-Jul-11
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)
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
: C
COMP2
is needed only for high ESR output capacitor
⎞
⎛
1.1
×
10
−
6
,0.012
×
V
OUT
⎟
≥
Min
⎜
⎟
⎜
C
OUT
⎠
⎝
(Ω)
(15)
The feedback loop of the IC is stabilized by the
components at the COMP pin, as shown in Figure
3. The DC loop gain of the system is determined by
the following equation:
And the proper value for C
COMP2
is:
C
COMP 2
=
C
OUT
R
ESRCOUT
R
COMP
(16)
A
VDC
0 . 808 V
=
A
VEA
G
COMP
I
OUT
(7)
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:
I
OUT
f
P 2
=
2
π
V
OUT
C
OUT
The first zero Z1 is due to R
COMP
and C
COMP
:
1
f
Z1
=
2
π
R
COMP
C
COMP2
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.
Table 1:
(8)
(9)
Typical Compensation for Different Output
Voltages and Output Capacitors
V
OUT
2.5V
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
8.2kΩ
12kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
15kΩ
C
COMP
C
COMP2
2.2nF
1.5nF
1.5nF
1.5nF
1.8nF
2.7nF
15nF
22nF
27nF
None
None
None
None
None
None
47pF
47pF
47pF
(10)
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
(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 .808 V
(Ω)
(12)
: C
COMP2
is needed for high ESR output capacitor.
C
COMP2
≤
47pF is recommended.
CC Loop Stability
The constant-current control loop is internally
compensated over the 400mA-1500mA output
range. No additional external compensation is
required to stabilize the CC current.
=
2 . 75
×
10
8
V
OUT
C
OUT
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
R
COMP
−
5
Output Cable Resistance Compensation
To compensate for resistive voltage drop across the
charger's output cable, the ACT4514 integrates a
-8-
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(F)
(13)
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TM
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