Classification | PRIVATE

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

8 of 19

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ACT4533A/B

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

External High Voltage Bias Diode

It is recommended that an external High Voltage Bias

diode be added when the system has a 5 V fixed input

or the power supply generates a 5 V output. This helps

improve the efficiency of the regulator. The High

Voltage Bias diode can be a low cost one such as

IN4148 or BAT54.

Figure 4: External High Voltage Bias Diode

This diode is also recommended for high duty cycle

operation and high output voltage applications.

Input Capacitor

The input capacitor needs to be carefully selected to

maintain sufficiently low ripple at the supply input of the

converter. A low ESR capacitor is highly recommended.

Since large current flows in and out of this capacitor

during switching, its ESR also affects efficiency.

The input capacitance needs to be higher than 10 µF.

The best choice is the ceramic type, however, low ESR

tantalum or electrolytic types may also be used provided

that the RMS ripple current rating is higher than 50%

of the output current. The input capacitor should be

placed close to the IN and G pins of the IC, with the

shortest traces possible. In the case of tantalum or

electrolytic types, they can be further away if a small

parallel 0.1 µF ceramic capacitor is placed right next to

the IC.

Output Capacitor

The output capacitor also needs to have low ESR to

keep low output voltage ripple. The output ripple voltage

is:

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× (������

+

×

×

)

(6)

Where IOUTMAX is the maximum output current, KRIPPLE is

the ripple factor, RESR is the ESR of the output capacitor,

fSW is the switching frequency, L is the inductor value,

and COUT is the output capacitance. In the case of

ceramic output capacitors, RESR is very small and does

not contribute to the ripple. Therefore, a lower

capacitance value can be used for ceramic type. In the

case of tantalum or electrolytic capacitors, the ripple

is dominated by RESR multiplied by the ripple current. In

that case, the output capacitor is chosen to have

sufficiently low ESR.

For ceramic output capacitor, typically choose a

capacitance of about 22 µF. For tantalum or electrolytic

capacitors, choose a capacitor with less than

50 mΩ ESR.

Rectifier Diode

Use a Schottky diode as the rectifier to conduct current

when the High-Side Power Switch is off. The Schottky

diode must have current rating higher than the

maximum output current and a reverse voltage rating

higher than the maximum input voltage.

STABILITY COMPENSATION

Figure 5: Stability Compensation



: CCOMP2 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:

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=

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(7)

The dominant pole P1 is due to CCOMP:

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=

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2������ ������

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(8)

The second pole P2 is the output pole:

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=

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2������ ������

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(9)

The first zero Z1 is due to RCOMP and CCOMP:

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=

1

2������ ������

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(10)

And finally, the third pole is due to RCOMP and CCOMP2

(if CCOMP2 is used):

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=

1

2������ ������

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(11)

The following steps should be used to compensate

the IC:

