ACT4455

Rev 2, 21-Nov-12

Innovative Power

TM

- 6 -

www.active-semi.com

Copyright © 2012 Active-Semi, Inc.

APPLICATIONS INFORMATION

Output Voltage Setting

Figure 1:

Output Voltage Setting

Figure 1 shows the connections for setting the

output voltage. Select the proper ratio of the two

Over Current Protection Setting

The output over current threshold is calculated by:

It is recommended that 1% or 0.5% high-accuracy

current sensing resistor is selected to achieve high-

accuracy over current protection. Two over current

protection thresholds can be different based on

different current sensing resistance.

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:

6.5A, as shown in Electrical Characteristics Table.

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 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 0.1µF 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:

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

(4)

(

)

SW

IN

OUT

IN

OUT

PK

LPK

f

V

L

V

V

V

I

×

×

×

=

_

_

PK

LPK

LOADMAX

LPK

_

I

2

1

I

I

+

=

(5)

(3)

(

)

RIPPLE

LOADMAX

SW

IN

OUT

IN

OUT

K

I

f

V

V

V

V

L

_

×

=

⎟

⎠

⎞

⎜

⎝

⎛

−

=

1

V

808

.

0

V

R

R

OUT

2

FB

1

FB

(1)

(6)

PK

LPK

LIM

OUTMAX

I

2

1

I

I

_

_

=

SENSE

2

OCP

1

OCP

R

/

mV

116

I

I

=

=

(2)

(7)

ESR

RIPPLE

OUTMAX

RIPPLE

R

K

I

V

=

OUT

2

SW

IN

LC

f

28

V

×

+