ACT4523

Rev 4, 21-Jul-11

Active-Semi

Innovative Power

TM

- 7 -

www.active-semi.com

Copyright © 2011 Active-Semi, Inc.

(6)

ESR

RIPPLE

OUTMAX

RIPPLE

R

K

I

V

=

OUT

2

SW

IN

LC

f

28

V

×

+

APPLICATIONS INFORMATION CONT’D

The maximum output current is calculated as:

3.2A, as shown in Electrical Characteristics Table.

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.

Figure 4:

External High Voltage Bias Diode

This diode is also recommended for high duty cycle

operation and high output voltage applications.

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

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.

Rectifier 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.

(5)

PK

LPK

LIM

OUTMAX

I

2

1

I

I

_

_

=