ACT4523(2011) データシートの表示（PDF） - Active-Semi, Inc
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ACT4523 Datasheet PDF : 15 Pages
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Active-Semi
ACT4523
Rev 4, 21-Jul-11
STABILITY COMPENSATION
Figure 5:
Stability Compensation
c: CCOMP2 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
5. The DC loop gain of the system is determined by
the following equation:
A VDC
=
0 .808 V
I OUT
AVEA G COMP
(7)
The dominant pole P1 is due to CCOMP:
fP 1
=
G EA
2 π A VEA C COMP
(8)
The second pole P2 is the output pole:
fP 2
=
I OUT
2πVOUT C OUT
(9)
The first zero Z1 is due to RCOMP and CCOMP:
1
f Z 1 = 2 π R COMP C COMP
(10)
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
fP3
=
1
2πRCOMP CCOMP2
(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 RCOMP:
R COMP
=
2 πVOUT C OUT fSW
10 G EA G COMP × 0 .808 V
= 5.12 ×10 7VOUT COUT (Ω)
(12)
STEP 2. Set the zero fZ1 at 1/4 of the cross over
frequency. If RCOMP is less than 15kΩ, the equation
for CCOMP is:
2 .83 × 10 5
C COMP =
R COMP
(F)
(13)
If RCOMP is limited to 15kΩ, then the actual cross
over frequency is 6.58 / (VOUTCOUT). Therefore:
C COMP
=
6 . 45
× 10
V _ 6
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 CCOMP2 is required. The condition for using
CCOMP2 is:
) R ESRCOUT
≥(Min 1.77 ×10 _ 6
COUT
,0 .006
× VOUT
(Ω)
(15)
And the proper value for CCOMP2 is:
CCOMP 2
=
C R OUT ESRCOUT
RCOMP
(16)
Though CCOMP2 is unnecessary when the output
capacitor has sufficiently low ESR, a small value
CCOMP2 such as 100pF may improve stability against
PCB layout parasitic effects.
Table 1 shows some calculated results based on
the compensation method above.
Table 1:
Typical Compensation for Different Output
Voltages and Output Capacitors
VOUT
2.5V
3.3V
5V
2.5V
3.3V
5V
COUT
47μF Ceramic CAP
47μF Ceramic CAP
47μF Ceramic CAP
470μF/6.3V/30mΩ
470μF/6.3V/30mΩ
470μF/6.3V/30mΩ
RCOMP
5.6kΩ
6.2kΩ
8.2kΩ
39kΩ
45kΩ
51kΩ
CCOMP CCOMP2c
3.3nF None
3.3nF None
3.3nF None
22nF 47pF
22nF 47pF
22nF 47pF
c: CCOMP2 is needed for high ESR output capacitor.
CCOMP2 ≤ 47pF is recommended.
CC Loop Stability
The constant-current control loop is internally
compensated over the 1500mA-3000mA output
range. No additional external compensation is
required to stabilize the CC current.
Output Cable Resistance Compensation
To compensate for resistive voltage drop across the
charger's output cable, the ACT4523 integrates a
simple, user-programmable cable voltage drop
compensation using the impedance at the FB pin.
Use the curve in Figure 6 to choose the proper
feedback resistance values for cable compensation.
Innovative PowerTM
-8-
www.active-semi.com
Copyright © 2011 Active-Semi, Inc.
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