ACT4060
Rev8, 24-Jul-07
STABILITY COMPENSATION
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:
Figure 2:
Stability Compensation
1.8 10 5
CCOMP
(F)
(10)
RCOMP
If RCOMP is limited to 15kΩ, then the actual cross
over frequency is 3.4 / (VOUTCOUT). Therefore:
C
COMP 1.2105VOUTCOUT
(11)
(F)
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:
: 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 2.
The DC loop gain of the system is determined by
the following equation:
6
1.110
RESRCOUT Min
,0.012VOUT
(Ω)
(12)
COUT
1.3V
And the proper value for CCOMP2 is:
AVDC
AVEAGCOMP
(4)
IOUT
COUTRESRCOUT
CCOMP2
(13)
The dominant pole P1 is due to CCOMP
:
RCOMP
G EA
Though CCOMP2 is unnecessary when the output ca-
pacitor has sufficiently low ESR, a small value
CCOMP2 such as 100pF may improve stability against
PCB layout parasitic effects.
fP 1
(5)
(6)
2 π AVEA C COMP
The second pole P2 is the output pole:
IOUT
Table 2 shows some calculated results based on
the compensation method above.
fP 2
2πVOUT COUT
The first zero Z1 is due to RCOMP and CCOMP
:
Table 2:
1
Typical Compensation for Different Output
Voltages and Output Capacitors
fZ1
(7)
2πRCOMP CCOMP2
VOUT
2.5V
3.3V
5V
COUT
RCOMP CCOMP CCOMP2
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
22μF Ceramic
22μF Ceramic
22μF Ceramic
47μF SP CAP
47μF SP CAP
47μF SP CAP
8.2kΩ
12kΩ
15kΩ
15kΩ
15kΩ
15kΩ
2.2nF
1.5nF
1.5nF
1.5nF
1.8nF
2.7nF
15nF
22nF
27nF
None
None
None
None
None
None
1nF
1
fP3
(8)
2πRCOMP CCOMP2
2.5V
3.3V
5V
The following steps should be used to compensate
the IC:
STEP 1. Set the cross over frequency at 1/10 of the
2.5V 470μF/6.3V/30mΩ 15kΩ
3.3V 470μF/6.3V/30mΩ 15kΩ
switching frequency via RCOMP
:
1nF
2πVOUT COUT fSW
5V
470μF/6.3V/30mΩ 15kΩ
None
RCOMP
10GEAGCOMP 1.3V
: CCOMP2 is needed for high ESR output capacitor.
1.7 108VOUT COUT
Figure 3 shows an example ACT4060 application circuit generat-
ing a 2.5V/2A output.
(Ω)
(9)
but limit RCOMP to 15kΩ maximum.
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