EA4533BYH Datasheet, PDF (9/17 Pages) Active-Semi, Inc – Wide-Input Sensorless CC/CV Step-Down DC/DC Converter

ACT4533A/B
Rev 0, 14-Aug-13
STABILITY COMPENSATION
Figure 5:
Stability Compensation
c
c: C_COMP2 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
5. The DC loop gain of the system is determined by
the following equation:
A _VDC
=
0 .808 V
I _OUT
A_VEA G _COMP
(7)
The dominant pole P1 is due to C_COMP:
f_{P 1}
=
G _EA
2 π A _VEA C _COMP
(8)
The second pole P2 is the output pole:
f_{P 2}
=
I _OUT
2πV_OUT C _OUT
(9)
The first zero Z1 is due to R_COMP and C_COMP:
1
f _{Z 1} = _{2 π R COMP C COMP}
(10)
And finally, the third pole is due to R_COMP and
C_COMP2 (if C_COMP2 is used):
f_P3
=
1
2πR_COMP C_COMP2
(11)
The following steps should be used to compensate
the IC:
STEP 1. Set the cross over frequency at 1/10 of the
switching frequency via R_COMP:
R _COMP
=
2 πV_OUT C _OUT f_SW
10 G _EA G _COMP × 0 .808 V
= 5.12 ×10 ⁷V_OUT C_{OUT (Ω)}
(12)
STEP 2. Set the zero f_Z1 at 1/4 of the cross over
frequency. If R_COMP is less than 15kΩ, the equation
for C_COMP is:
2 .83 × 10 ⁵
C _COMP =
R _COMP
(F)
(13)
If R_COMP is limited to 15kΩ, then the actual cross
over frequency is 6.58 / (V_OUTC_OUT). Therefore:
C _COMP
=
6 . 45
× 10
V _ 6
OUT
C _OUT
(F)
(14)
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 C_COMP2 is required. The condition for using
C_COMP2 is:
) R _ESRCOUT
≥(Min ^{1.77 ×10 _ 6}
C_OUT
,0 .006
× V_OUT
(Ω)
(15)
And the proper value for C_COMP2 is:
C_{COMP 2}
=
C R _{OUT ESRCOUT}
R_COMP
(16)
Though C_COMP2 is unnecessary when the output
capacitor has sufficiently low ESR, a small value
C_COMP2 such as 100pF may improve stability against
PCB layout parasitic effects.
Table 1 shows some calculated results based on
the compensation method above.
Table 1:
Typical Compensation for Different Output
Voltages and Output Capacitors
V_OUT
2.5V
3.3V
5V
2.5V
3.3V
5V
C_OUT
47μF Ceramic CAP
47μF Ceramic CAP
47μF Ceramic CAP
220μF/10V/30mΩ
220μF/10V/30mΩ
220μF/10V/30mΩ
R_COMP
5.6kΩ
8.2kΩ
15kΩ
15kΩ
15kΩ
15kΩ
C_COMP C_COMP2^c
10nF None
10nF None
10nF None
2.2nF 47pF
2.2nF 47pF
2.2nF 47pF
c: C_COMP2 is needed for high ESR output capacitor.
C_COMP2 ≤ 47pF is recommended.
CC Loop Stability
The constant-current control loop is internally
compensated over the 1500mA-3500mA output
range. No additional external compensation is
required to stabilize the CC current.
Output Cable Resistance Compensation
To compensate for resistive voltage drop across the
charger's output cable, the ACT4533A/B integrates
a simple, user-programmable cable voltage drop
compensation using the impedance at the FB pin.
Use the curve in Figure 6 to choose the proper
feedback resistance values for cable compensation.
R_FB1 is the high side resistor of voltage divider.
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