3A, 18V, Synchronous Step-down DC/DC Converter

EC3293

Page 8 of 11

3L03N-Rev.P001

Application Information(Cont.)

ripple current that will result in lower output ripple

voltage. However, the larger value inductor will have a

larger physical size, higher series resistance, and/or

lower saturation current. A good rule for determining

the inductance to use is to allow the peak-to-peak ripple

current in the inductor to be approximately 30% of the

maximum switch current limit. Also, make sure that the

peak inductor current is below the maximum switch

current limit. The inductance value can be calculated by:

inductor ripple current.

Choose an inductor that will not saturate under the

maximum inductor peak current. The peak inductor

current can be calculated by:

The choice of which style inductor to use mainly

depends on the price vs. size requirements and any EMI

requirements.

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. Table 1 lists example

Schottky diodes and their Manufacturers.

Table 1. Diode Selection guide

Input Capacitor

The input current to the step-down converte is

discontinuous, therefore a capacitor is required

to supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low ESR

capacitors for the best performance. Ceramic capacitors

are preferred,but tantalum or low-ESR electrolytic

capacitors may also suffice. Choose X5R or X7R

dielectrics when using ceramic capacitors.

Since

the

input

capacitor

(C1)

absorbs

the

input

switching current it requires an adequate ripple current

rating. The RMS current in the input capacitor can be

estimated by:

1/2

whose RMS current rating greater than half of the

maximum load current.

The input capacitor can be electrolytic, tantalum or

ceramic. When using electrolytic or tantalum capacitors,

a small, high quality ceramic capacitor, i.e.

0.1μF, should

be placed as close to the IC as possible. When using

ceramic capacitors, make sure that they have enough

capacitance to provide sufficient charge to prevent

excessive voltage ripple at input. The input voltage

ripple for low ESR capacitors can be estimated by:

ΔV

Where C1 is the input capacitance value.

Output Capacitor

The output capacitor is required to maintain the DC

output

voltage.

Ceramic,

tantalum,

or

low

ESR

electrolytic capacitors are recommended. Low ESR

capacitors are preferred to keep the output voltage

ripple low. The output voltage ripple can be estimated

by:

ΔV

equivalent series resistance (ESR) value of the output

capacitor.

In the case of ceramic capacitors, the impedance at the

switching frequency is dominated by the capacitance.

The output voltage ripple is mainly caused by the

capacitance. For simplification, the output voltage ripple

can be estimated by:

ΔV

2

× L × C2) ] × (1 − V

In the case of tantalum or electrolytic capacitors, the

ESR

dominates

the

impedance

at

the

switching

frequency. For simplification, the output ripple can be

approximated to:

ΔV

The characteristics of the output capacitor also affect

the stability of the regulation system. The EC3293

Part

Number

Voltage and

Current Rating

Vendor

B130

30V, 1A

Diodes Inc.

SK13

30V, 1A

Diodes Inc.

MBRS130

30V, 1A

International

Rectifier