Data Sheet Rev. B, September 2019 | Subject to change without notice

8 of 15

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ACT4530M

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

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:

=

×

−

(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 currents flow 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 a ceramic capacitor. 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.

Active

Semi

recommends using a ceramic capacitor in parallel with

a tantalum or electrolytic. This combination provides the

EMI and noise performance. The input capacitor must

be placed close to the IN and GND pins of the IC, with

the shortest traces possible. If using a tantalum or elec-

trolytic capacitor in parallel with ceramic capacitor, the

ceramic capacitor must be placed closer to the IC.

Output Capacitor

The ACT4530 output capacitance must be split between

the left and right side of the output current sense resistor.

The left side of the current sense resistor (CSP pin)

requires a 22 uF ceramic capacitor. The right side of the

current

sense

resistor

should

contain

enough

capacitance to keep the output voltage ripple below the

required level.

=

×

+

!

" × #$% × &'()

)

(5)

This output capacitance should have low ESR to keep

low output voltage ripple. The output ripple voltage is:

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. In the case of

tantalum or electrolytic capacitors, the ripple is

dominated by RESR. In this case, the output capacitor

must be chosen to have sufficiently low ESR.

For ceramic output capacitors, 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 and the output voltage ripple is dominated

by ESR, add a 2.2 uF ceramic in parallel with the

tantalum or electrolytic.

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.

APPLICATIONS INFORMATION

Current Sense Resistor

The traces leading to and from the sense resistor can

be significant error sources. With small value sense