ACT4012A
Rev PrA, 21-Nov-07
STEP 2. Set the zero fZ1 at 1/4 of the cross over
frequency. If RCOMP is less than 15kꢀ, the equation
for CCOMP is:
STABILITY COMPENSATION
Figure 2:
1.8 ×10 −5
RCOMP
Stability Compensation
CCOMP
=
(F)
(10)
If RCOMP is limited to 15kꢀ, then the actual cross
over frequency is 3.4 / (VOUTCOUT). Therefore:
CCOMP =1.2×10−5VOUTCOUT
(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:
c: CCOMP2 is needed only for high ESR output capacitor
The feedback system 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
⎛
⎞
1.1×10
⎜
⎟
⎟
RESRCOUT ≥ Min
,0.012×VOUT
(Ω)
(12)
⎜
COUT
⎝
⎠
And the proper value for CCOMP2 is:
1.3V
AVDC
=
AVEAGCOMP
(4)
COUTRESRCOUT
IOUT
(13)
CCOMP2
=
RCOMP
The dominant pole P1 is due to CCOMP
:
GEA
Though CCOMP2 is unnecessary when the output ca-
pacitor has sufficiently low ESR, a small value
fP1
=
(5)
(6)
2πAVEA CCOMP
C
COMP2 such as 100pF may improve stability against
The second pole P2 is the output pole:
PCB layout parasitic effects.
IOUT
Table 2 shows some calculated results based on
the compensation method above.
fP 2
=
2πVOUT COUT
Table 2:
The first zero Z1 is due to RCOMP and CCOMP
:
Typical Compensation for Different Output
Voltages and Output Capacitors
1
fZ1
=
(7)
2πRCOMPCCOMP2
c
VOUT
2.5V
3.3V
5V
COUT
RCOMP CCOMP CCOMP2
And finally, the third pole is due to RCOMP and
22µF Ceramic
22µF Ceramic
22µF Ceramic
47µF SP CAP
47µF SP CAP
47µF SP CAP
12kꢀ
15kꢀ
15kꢀ
15kꢀ
15kꢀ
15kꢀ
1.5nF
1.5nF
1.5nF
1.5nF
1.8nF
2.7nF
15nF
22nF
27nF
None
None
None
None
None
None
1nF
C
COMP2 (if CCOMP2 is used):
1
fP3
=
(8)
2πRCOMP CCOMP2
2.5V
3.3V
5V
Follow the following steps to compensate the IC:
STEP 1. Set the cross over frequency at 1/10 of the
switching frequency via RCOMP
:
2.5V 470µF/6.3V/30mꢀ 15kꢀ
3.3V 470µF/6.3V/30mꢀ 15kꢀ
2πVOUT COUT fSW
1nF
RCOMP
=
5V
470µF/6.3V/30mꢀ 15kꢀ
None
10GEAGCOMP ×1.3V
= 2 × 10 8 VOUT COUT
c: CCOMP2 is needed for high ESR output capacitor.
(Ω)
(9)
Figure 3 shows an example ACT4012A application circuit gener-
ating a 5V/1.5A output.
but limit RCOMP to 15kꢀ maximum.
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