ACT4525

- 7 -

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Copyright © 2015 Active-Semi, Inc.

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:

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 maximum output current is calculated as:

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 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:

capacitance. In the case of ceramic output

contribute to the ripple. Therefore, a lower

capacitance value can be used for ceramic type. In

the case of tantalum or electrolytic capacitors, the

current. In that case, the output capacitor is chosen

to have sufficiently low ESR.

For ceramic output capacitor, typically choose a

capacitance of about 22µF. For tantalum or

electrolytic capacitors, choose a capacitor with less

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

capacitor 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. Further more, the low forward voltage

Schottky is preferable for high efficiency and

smoothly operation.

Current Sense Resistor

The traces leading to and from the sense resistor

can be significant error sources. With small value

sense resistors, trace resistance shared with the

load

can

cause

significant

errors.

It

is

recommended to connect the sense resistor pads

directly to the CSP and CSN pins using “Kelvin” or

“4-wire” connection techniques as shown below.

(1)

(

)

RIPPLE

LOADMAX

SW

IN

OUT

IN

OUT

K

I

f

V

V

V

V

L

_

×

=

(2)

(

)

SW

IN

OUT

IN

OUT

PK

LPK

f

V

L

V

V

V

I

×

×

×

=

_

_

PK

LPK

LOADMAX

LPK

_

I

2

1

I

I

+

=

(3)

(4)

PK

LPK

LIM

OUTMAX

I

2

1

I

I

_

_

=

(5)

OUT

SW

IN

ESR

RIPPLE

OUTMAX

RIPPLE

LC

f

V

R

K

I

V

2

28 





PCB Load

Trace

Kevin Sense

Traces

Sense

Resistor