ACT4513
Active- Semi
Rev 6, 29-Jul-11
STABILITY COMPENSATION
If RCOMP is limited to 15kꢀ, then the actual cross
over frequency is 3.4 / (VOUTCOUT). Therefore:
Figure 5:
Stability Compensation
CCOMP =1.2×10−5VOUTCOUT
(14)
(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:
−6
⎛
⎞
1.1×10
⎜
⎜
⎟
⎟
RESRCOUT ≥ Min
,0.012 ×VOUT
(Ω)
(15)
(16)
COUT
⎝
⎠
c: CCOMP2 is needed only for high ESR output capacitor
And the proper value for CCOMP2 is:
The feedback loop of the IC is stabilized by the
components at the COMP pin, as shown in Figure
3. The DC loop gain of the system is determined by
the following equation:
COUT RESRCOUT
CCOMP2
=
RCOMP
Though CCOMP2 is unnecessary when the output
capacitor has sufficiently low ESR, a small value
0.808V
AVDC
=
AVEA GCOMP
(7)
IOUT
CCOMP2 such as 100pF may improve stability against
PCB layout parasitic effects.
The dominant pole P1 is due to CCOMP
:
Table 2 shows some calculated results based on
the compensation method above.
G
EA
fP 1
=
(8)
(9)
2 π A VEA
C
COMP
Table 1:
The second pole P2 is the output pole:
Typical Compensation for Different Output
Voltages and Output Capacitors
IOUT
fP 2
=
2πVOUT COUT
c
VOUT
COUT
RCOMP CCOMP CCOMP2
The first zero Z1 is due to RCOMP and CCOMP
:
2.5V 47μF Ceramic CAP 5.6kꢀ 3.3nF
3.3V 47μF Ceramic CAP 6.2kꢀ 3.3nF
None
None
None
47pF
47pF
47pF
1
fZ 1
=
(10)
2π R COMP C COMP
5V
2.5V
3.3V
5V
47μF Ceramic CAP 8.2kꢀ 3.3nF
And finally, the third pole is due to RCOMP and
CCOMP2 (if CCOMP2 is used):
470μF/6.3V/30mꢀ
470μF/6.3V/30mꢀ
470μF/6.3V/30mꢀ
39kꢀ
45kꢀ
51kꢀ
22nF
22nF
22nF
1
fP3
=
(11)
2πRCOMPCCOMP2
c: CCOMP2 is needed for high ESR output capacitor.
CCOMP2 ≤ 47pF is recommended.
The following steps should be used to compensate
the IC:
CC Loop Stability
STEP 1. Set the cross over frequency at 1/10 of the
The constant-current control loop is internally
compensated over the 750mA-2500mA output
range. No additional external compensation is
required to stabilize the CC current.
switching frequency via RCOMP
:
2πVOUT COUT fSW
RCOMP
=
10GEAGCOMP × 0.808V
= 2.75 ×10 8VOUT COUT
(12)
(Ω)
Output Cable Resistance Compensation
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:
To compensate for resistive voltage drop across the
charger's output cable, the ACT4513 integrates a
simple, user-programmable cable voltage drop
compensation using the impedance at the FB pin.
Use the curve in Figure 4 to choose the proper
feedback resistance values for cable compensation.
1.8 ×10 −5
CCOMP
=
(F)
(13)
RCOMP
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