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ACT4070BYH-T Datasheet PDF : 9 Pages
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ACT4070B
Rev 0, 23-Apr-12
Stability compensation
Figure 2:
Stability Compensation
c
c: CCOMP2 is needed only for high ESR output capacitor
The feedback system of the IC is stabilized by the
components at COMP pin, as shown in Figure 2.
The DC loop gain of the system is determined by
the following equation:
AVDC
= 0 .808 V
IOUT
AVEA G COMP
(4)
The dominant pole P1 is due to CCOMP:
fP1
=
G EA
2 π AVEA C COMP
(5)
The second pole P2 is the output pole:
fP 2
=
IOUT
2 πVOUT 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 1/10 of
the switching frequency via RCOMP:
R COMP
=
2 π V OUT C OUT fSW
10 G EA G COMP 0 .808 V
(Ω)
(9)
=
5 .12
x10
V 7
OUT
C OUT
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:
C COMP
= 2 . 83 x 10 5
R COMP
(F)
(10)
If RCOMP is limited to 15kΩ, then the actual cross
over frequency is 4.8/(VOUTCOUT). Therefore:
C COMP
=
6 .45
x10
V − 6
OUT
C OUT
(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 us-
ing CCOMP2 is:
R ESROUT
≥
Min
⎜⎜⎝⎛
1
.
77
C
x10
OUT
−6
,0 .006 VOUT
⎟⎟⎠⎞
(Ω)
(12)
And the proper value for CCOMP2 is:
CCOMP
= C R OUT ESROUT
RCOMP
(13)
Though CCOMP2 is unnecessary when the output
capacitor has sufficiently low ESR, a small value
CCOMP2 such as 220pF 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
1.8V
COUT
22μF Ceramic
RCOMP
4kΩ
CCOMP CCOMP2c
3.3nF 220pF
2.5V 22μF Ceramic 5.6kΩ 3.3nF 220pF
5V 22μF Ceramic 12kΩ 1.5nF 220pF
1.8V 100μF SP CAP 15kΩ 1.5nF 220pF
2.5V 100μF SP CAP 15kΩ 2.2nF 220pF
5V 100μF SP CAP 15kΩ 4.7nF 220pF
c: CCOMP2 is needed for board parasitic and high ESR output
capacitor.
Figure 3 shows a sample ACT4070B application
circuit generating a 2.5V/3A output.
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Copyright © 2012 Active-Semi, Inc.
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