ACT4065SH-T Ver la hoja de datos (PDF) - Active-Semi, Inc
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ACT4065SH-T Datasheet PDF : 9 Pages
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ACT4065
STABILITY COMPENSATION
COMP
ACT4065
CC OMP
RC OMP
CC
O
MP
*
2
*CCOMP2 is needed only for high ESR output capacitor
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:
AVDC
=
1.293V
IOUT
AVEAGCOMP
(4)
The dominant pole P1 is due to CCOMP:
fP1
=
GEA
2πAVEA CCOMP
(5)
The second pole P2 is the output pole:
fP 2
=
IOUT
2πVOUT COUT
(6)
The first zero Z1 is due to RCOMP and CCOMP:
fZ1 =
1
2πRCOMP CCOMP
(7)
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
fP 3
=
1
2πRCOMP CCOMP2
(8)
Follow the following steps to compensate the
IC:
STEP 1. Set the cross over frequency at 1/5 of
the switching frequency via RCOMP:
RCOMP
=
2πVOUTCOUT fSW
10GEAGCOMP •1.293V
= 9.8 × 107VOUTCOUT
(Ω )
(9)
but limit RCOMP to 15kΩ maximum.
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:
CCOMP
=
1.6 × 10 − 5
RCOMP
(F )
(10)
If RCOMP is limited to 15kΩ, then the actual cross
over frequency is 6.1 / (VOUTCOUT). Therefore:
CCOMP
=
6.96 ×
10
−
V6
OUT
COUT
(F )
(11)
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
1.1 × 10
COUT
−
6
,0.012 • VOUT
( Ω ) (12)
And the proper value for CCOMP2 is:
CCOMP 2
=
COUT RESRCOUT
RCOMP
(13)
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 2 shows some calculated results based
on the compensation method above.
Table 2. Typical Compensation for Different
Output Voltages and Output Capacitors
VOUT
COUT
RCOMP
2.5V 22μF Ceramic 5.6kΩ
3.3V 22μF Ceramic 7.2kΩ
5V 22μF Ceramic 10kΩ
2.5V 47μF SP Cap 11kΩ
3.3V 47μF SP Cap 15kΩ
5V
47μF SP Cap 15kΩ
2.5V 470μF/6.3V/30mΩ 15kΩ
3.3V 470μF/6.3V/30mΩ 15kΩ
5V 470μF/10V/30mΩ 15kΩ
CCOMP
2.7nF
2.2nF
1.5nF
1.5nF
1nF
1.5nF
8.2nF
10nF
15nF
CCOMP2
None
None
None
None
None
None
1nF
1nF
None
Figure 5 shows a sample ACT4065
application circuit generating 2.5V/2A output.
Active-Semi, Inc.
-6-
www.active-semi.com
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