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3G16N-Rev.P001

2.2A/32V Synchronous Rectified

Step-Down Converter

EC3276

Function Description

Component Selection

Setting the Output Voltage

The output voltage is set using a resistive voltage divider

from the output voltage to FB pin. The voltage divider

divides the output voltage down to the feedback voltage by

the ratio:

voltage. Thus the output voltage is:

R2 can be as high as 100kΩ, but a typical value is 10kΩ.

Using the typical value for R2, R1 is determined by:

For example, for a 3.3V output voltage, R2 is 10kΩ, and R1

is 26.1kΩ.

Inductor

The inductor is required to supply constant current to the

output load while being driven by the switched input voltage.

A larger value inductor will result in less 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:

Where VOUT is the output voltage, VIN is the input voltage,

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 lowside 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：

Part Number

Voltage/Current

Vendor

B140

40V, 1A

Diodes, Inc.

SK14

40V, 1A

Diodes, Inc.

MBRS140

40V, 1A

International Rectifier

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.