ACT4501 datasheet(7/16 Pages) ACTIVE-SEMI

7 / 16 page

ACT4501

Rev 2, 14-Nov-12

Innovative Power

- 7 -

www.active-semi.com

(6)

ESR

RIPPLE

OUTMAX

RIPPLE

OUT

APPLICATIONS INFORMATION CONT’D

Figure 3:

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:

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

inductor value, and COUT is the output capacitance.

In the case of ceramic output capacitors, RESR is very

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

RESR multiplied by the ripple 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.

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