Classification | PRIVATE

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

7 of 19

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ACT4533A/B

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

APPLICATIONS INFORMATION

Output Voltage Setting

Figure 1: Output Voltage Setting

Figure 1 shows the connections for setting the output

voltage. Select the proper ratio of the two feedback

resistors RFB1 and RFB2 based on the output voltage.

Adding a capacitor in parallel with RFB1 helps the system

stability. Typically, use RFB2 ≈ 10 kΩ and determine

RFB1 from the following equation:

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= ������

������

0.808������

− 1

(1)

CC Current Setting

ACT4533A/B constant current value is set by a resistor

connected between the ISET pin and GND. The CC

output current is linearly proportional to the current

flowing out of the ISET pin. The voltage at ISET is roughly

1.1 V and the current gain from ISET to output is roughly

21000 (21 mA/1 µA). To determine the proper resistor

for a desired current, please refer to Figure 2 below.

Figure 2: Curve for Programming Output CC Current

CC Current Line Compensation

When operating at constant current mode, the current

limit increases slightly with input voltage. For wide input

voltage applications, a resistor RC may be added

to compensate line change and keep output high CC

accuracy, as shown in Figure 3.

Figure 3: Input Line Compensation

Inductor Selection

The inductor maintains a continuous current to the

output load. This inductor current has a ripple that is

dependent on the inductance value:

Higher inductance reduces the peak-to-peak ripple

current. The trade-off for high inductance value is

the increase in inductor core size and series resistance,

and the reduction in current handling capability. In

general, select an inductance value L based on ripple

current requirement:

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������

× (������ − ������

)

������ ������

������

������

(2)

where VIN is the input voltage, VOUT is the output voltage,

fSW is the switching frequency, ILOADMAX is the maximum

load current, and KRIPPLE is the ripple factor. Typically,

choose KRIPPLE = 30% to correspond to the peak-to-peak

ripple current being 30% of the maximum load current.

With a selected inductor value, the peak-to-peak

inductor current is estimated as:

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=

������

× (������ − ������

)

������ × ������ × ������

(3)

The peak inductor current is estimated as:

������

= ������

+

1

2

������

(4)

The selected inductor should not saturate at ILPK.

The maximum output current is calculated as:

������

= ������

−

1

2

������

(5)

LLIM is the internal current limit, which is typically 4.5 A,

as shown in the Electrical Characteristics Table.