EC3293B datasheet(9/11 Pages) E-CMOS

9 / 11 page

EC3293B

3A, 18V, 500KHz,

Synchronous Step Down DC/DC Converter

E-CMOS Corp. (www.ecmos.com.tw)

5G01N-Rev. F002

9/11

Input Capacitor

The input current to the step-down converter 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:

IC1 = ILOAD × [ (VOUT/VIN) × (1 − VOUT/VIN) ]

1/2

The worst-case condition occurs at VIN = 2VOUT, where IC1= ILOAD/2. For simplification, choose the input

capacitor 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

provide

sufficient

charge

prevent excessive

voltage

ripple

input.

The

input

voltage ripple for low ESR

capacitors can be estimated by:

ΔV

IN = [ ILOAD/(C1 × fS) ] × (VOUT/VIN) × (1 − VOUT/VIN)

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:

ΔVOUT = [ VOUT/(fS × L) ] × (1 − VOUT/VIN)× [ RESR + 1 / (8 × fS × C2) ]

Where C2 is the output capacitance value and RESR is the 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

mainly

caused

the capacitance. For simplification, the output voltage

ripple can be estimated by:

ΔVOUT = [ VOUT/(8xfS

2 xLxC2)] × (1 − V

OUT/VIN)

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:

ΔVOUT = [ VOUT/(fS × L) ] × (1 − VOUT/VIN) × RESR

The characteristics of the output capacitor also affect the stability of the regulation system. The EC3293B

can be optimized for a wide range of capacitance and ESR values.

External Bootstrap Diode

An external bootstrap diode may enhance the efficiency of the regulator, the applicable conditions of

external

BOOT diode are:

● VOUT = 5V or 3.3V; and

● Duty cycle is high: D = VOUT/VIN > 65%

Figure 2: Add optional external bootstrap diode to enhance efficiency.

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