ACT2102

Rev 2, 23-May-12

®

- 8 -

www.active-semi.com

Copyright © 2012 Active-Semi, Inc.

(6)

ESR

RIPPLE

OUTMAX

RIPPLE

R

K

I

V

=

OUT

2

SW

IN

LC

f

28

V

×

+

APPLICATIONS INFORMATION CONT’D

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 2:

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 GND 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:

small and does not contribute to 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

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.

Optional Schottky Diode

During the transition between high-side switch and

low-side switch, the body diode of the low-side

power MOSFET conducts the inductor current. The

forward voltage of this body diode is high. An

optional Schottky diode may be paralleled between

the SW pin and GND pin to improve overall

efficiency.