ACT4524 datasheet(7/16 Pages) ACTIVE-SEMI

7 / 16 page

ACT4524

Rev 0, 28-Jul-15

Innovative Power

- 7 -

www.active-semi.com

APPLICATIONS INFORMATION

Inductor Selection

The inductor maintains a continuous current to the

output load. This inductor current has a ripple which

is determined by 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:

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:

The peak inductor current is estimated as:

The selected inductor should not saturate at ILPK.

The maximum output current is calculated as:

LLIM is the internal current limit.

External High Voltage Bias Diode

It is recommended that an external High Voltage

Bias diode be added when the system has a 5V

fixed input or the power supply generates a 5V

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.

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

10µF. 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 VIN and GND pins of the IC, with the shortest

traces as possible. In the case of tantalum or

electrolytic types, they can be placed a little bit

away of IC if a

paralleled 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:

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 inductance, and COUT is the output capacitance.

In the case of ceramic output capacitors, RESR is very

small and only contributes a very small portion of 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 should be chosen to have

sufficiently low ESR.

For ceramic type output capacitor, typically choose

a capacitance of about 22µF. For tantalum or

electrolytic capacitors, choose a capacitor with less

than 50mΩ ESR. A 330µF or 470µF electrolytic

capacitor is recommended.

Rectifier Diode

Use a low forward voltage drop (Vf<0.5V) 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.

(1)

(

)

RIPPLE

LOADMAX

OUT

(2)

(

)

OUT

LPK

LOADMAX

LPK

(3)

(4)

LPK

LIM

OUTMAX

(5)

ESR

RIPPLE

OUTMAX

RIPPLE



OUT





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