ACT4072SH-T データシートの表示（PDF） - Active-Semi, Inc
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ACT4072SH-T Datasheet PDF : 10 Pages
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Active-Semi
ACT4072
Rev2, 27-May-08
Stability compensation
Figure 2:
Stability Compensation
COMP
ACT4072
CCOMP
RCOMP
CCOPM2c
c: CCOMP2 is needed only for high ESR output capacitors or PCB
parasitics
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:
AVDC
= 1.222V
IOUT
AVEAGCOMP
(4)
The dominant pole P1 is due to CCOMP:
fP1
=
GEA
2πAVEACCOMP
(5)
The second pole P2 is the output pole:
fP 2
=
IOUT
2πVOUTCOUT
(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:
RCOMP
= 2πVOUT COUT fSW
10GEAGCOMP1.222V
=
2.3
x10
V 8
OUT
COUT
(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
= 1.8 ×10 −5
RCOMP
(F) (10)
If RCOMP is limited to 15kΩ, then the actual cross
over frequency is 3.4/(VOUTCOUT). Therefore:
CCOMP
=
1.3
x10
V C −5
OUT 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 using
CCOMP2 is:
RESROUT
≥
Min⎜⎜⎝⎛
1.1x10
COUT
−6
,0.012VOUT
⎟⎟⎠⎞
(Ω)
(12)
And the proper value for CCOMP2 is:
CCOMP2
=
C R OUT ESROUT
RCOMP
(13)
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 volt-
ages and Output Capacitors
VOUT
COUT
RCOMP CCOMP CCOMP2c
2.5V 22µF Ceramic 8.2kΩ 2.2nF 100pF
3.3V 22µF Ceramic 12kΩ 1.5nF 100pF
5V
22µF Ceramic 15kΩ 1.5nF 100pF
2.5V 47µF SP CAP 15kΩ 1.5nF 100pF
3.3V 47µF SP CAP 15kΩ 1.8nF 100pF
5V
47µF SP CAP 15kΩ 2.7nF 100pF
2.5V 470µF/6.3V/30mΩ 15kΩ 15nF 1nF
3.3V 470µF/6.3V/30mΩ 15kΩ 22nF 1nF
5V 470µF/6.3V/30mΩ 15kΩ 27nF 1nF
c: CCOMP2 is needed for board parasitic and high ESR output
capacitor.
Figure 3 shows an example ACT4072 application
circuit generating a 5V/2A output.
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