ACT4065A Ver la hoja de datos (PDF) - Unspecified
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ACT4065A Datasheet PDF : 9 Pages
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®
ACT4065A
Rev 0, 23-Apr-12
Stability compensation
Figure 2:
Stability Compensation
c: CCOMP2 is needed only for high ESR output capacitors
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:
A VDC
=
0 .808 V
I OUT
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
2π VOUT 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:
R COMP
=
2 π VOUT C OUT fSW
10 G EA G COMP × 0 .808 V
=
2 .75
× 10
V 8
OUT
C OUT
(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:
C COMP
= 1 . 8 × 10 − 5
R COMP
(F) (10)
If RCOMP is limited to 15kΩ, then the actual cross
over frequency is 6.1/ (VOUTCOUT). Therefore:
C COMP
=
1.2
× 10
V 5
OUT
C OUT
(F) (11)
STEP 3. If the output capacitors 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
required. The condition for using CCOMP2 is:
RESROUT
≥
Min⎜⎜⎝⎛
1.1 ×10
COUT
−6
,0.012VOUT ⎟⎟⎠⎞
(Ω)
(12)
And the proper value for CCOMP2 is:
CCOMP 2
=
COUT RESROUT
RCOMP
(13)
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.
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
2.5V
COUT
22μF Ceramic
RCOMP
12kΩ
CCOMP
2.2nF
CCOMP2c
None
3.3V 22μF Ceramic 12kΩ 1.5nF None
5V 22μF Ceramic 15kΩ 2.2nF None
2.5V 47μF SP Cap 15kΩ 1.5nF None
3.3V 47μF SP Cap 15kΩ 1.8nF None
5V
47μF SP Cap 15kΩ 2.7nF None
2.5V 470µF/6.3V/30mΩ 15kΩ 1.5nF 47pF
3.3V 470µF/6.3V/30mΩ 15kΩ 2.2nF 47pF
5V 470µF/10V/30mΩ 15kΩ 2.7nF 47pF
c: CCOMP2 is needed only for high ESR output capacitors
Figure 3 shows a sample ACT4065A application
circuit generating a 2.5V/2A output.
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