Classification | PRIVATE

Data Sheet Rev. B, January 2025 | Subject to change without notice

9 of 19

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ACT4533A/B

Wide Input Sensorless CC/CV Step-Down DC/DC Converter

STEP 1. Set the cross over frequency at 1/10 of the

switching frequency via RCOMP:

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=

2������ ������

������

������

10 ������

������

× 0.808������

= 5.12 ������ 10 ������

������

(������)

(12)

STEP 2. Set the zero fZ1 at 1/4 of the cross over

frequency. If RCOMP is less than 15 kΩ, the equation for

CCOMP is:

������

=

2.83 × 10

������

(������)

(13)

If RCOMP is limited to 15 kΩ, then the actual crossover

frequency is 6.58 / (VOUTCOUT). Therefore:

������

= 6.45 × 10

������

������

(������)

(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

CCOMP2 is required. The condition for using CCOMP2 is:

������

≥ ������������������

1.77 × 10

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, 0.006 × ������

(������) (15)

And the proper value for CCOMP2 is:

������

=

������

������

������

(16)

Though CCOMP2 is unnecessary when the output

capacitor has sufficiently low ESR, a small value CCOMP2

such as 100 pF 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

VOUT

COUT

RCOMP

VCOMP CCOMP2

2.5 V

47 μF Ceramic CAP

5.6 kΩ

10 nF

None

3.3 V

47 μF Ceramic CAP

8.2 kΩ

10 nF

None

5 V

47 μF Ceramic CAP

15 kΩ

10 nF

None

2.5 V

220 μF/10 V/30 mΩ

15 kΩ

2.2 nF

47 pF

3.3 V

220 μF/10 V/30 mΩ

15 kΩ

2.2 nF

47 pF

5 V

220 μF/10 V/30 mΩ

15 kΩ

2.2 nF

47 pF



: CCOMP2 is needed for high ESR output capacitor.

CCOMP2 ≤ 47 pF is recommended.

CC Loop Stability

The

constant-current

control

loop

is

internally

compensated over the 1500 mA - 3500 mA 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. RFB1 is the

high side resistor of voltage divider.

In the case of high RFB1 used, the frequency compensa-

tion needs to be adjusted correspondingly. As shown in

Figure 7, adding a capacitor in parallel with RFB1 or in-

creasing the compensation capacitance at COMP pin

helps the system stability.

Figure 6: Cable Compensation at Various Resistor

Divider Values

Figure 7: Frequency Compensation for High RFB1

PC Board Layout Guidance

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of

the IC.