Data Sheet Rev. B, September 2020 | Subject to change without notice

8 of 16

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ACT4529M

40 V/3.0 A CV/CC Buck Converter Featuring QC2.0, USB Auto-Detect and USB-PD

APPLICATIONS INFORMATION

Inductor Selection

The inductor maintains a continuous current to the out-

put 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:

=

×

−

(1)

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:

=

×

−

×

×

(2)

The peak inductor current is estimated as:

=

+

1

2

(3)

The selected inductor should not saturate at ILPK. The

maximum output current is calculated as:

=

−

1

2

(4)

LLIM is the internal current limit.

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 GND pins of the IC, with the shortest

traces possible. In the case of tantalum or electrolytic

types, a ceramic capacitor is recommended to parallel

with tantalum or electrolytic capacitor, which should be

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:

=

×

+

!

" × #$% × &'()

)

(5)

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.

From the equation above, VRIPPLE is the combination of

ESR and real 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. If an 330 uF or 470 uF electrolytic cap or tantalum

cap is used, where ripple is dominantly caused by ESR,

an 2.2 uF ceramic in parallel is recommended.

Rectifier Schottky 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. Furthermore,

the low forward voltage Schottky is preferable for high

efficiency and smoothly operation.