(duty cycle = 50%)

IOUTMAX

2

IRMS =

IRMS = IOUTMAX x

x

VOUT

VIN

1-

©

§

VOUT

VIN

L t

VOUT

VIN x f

©

§

x

VIN - VOUT

IPP

©

§

LP3923

www.ti.com

SNVS567B – APRIL 2010 – REVISED MAY 2013

ILOAD = load current

IRIPPLE = average to peak inductor current

VIN

= input voltage

L

= inductor inductance

f

= switching frequency

Method 2

A more conservative approach is to choose an inductor that can handle the maximum current limit of 1500 mA.

Given a peak-to-peak current ripple (IPP) the inductor needs to be at least:

(4)

A 2.2 µH inductor with a saturation current rating of at least 1500 mA is recommended for most applications. The

inductor’s resistance should be less than 0.3

Ω for good efficiency. The below table suggests inductors and

suppliers.

For low-cost applications, an unshielded bobbin inductor is suggested. For noise critical applications, a toroidal or

shielded-bobbin inductor should be used. A good practice is to lay out the board with overlapping footprints of

both types for design flexibility. This allows substitution of a low-noise toroidal inductor, in the event that noise

from low-cost bobbin models is unacceptable.

Table 16. Suggested Inductors and Their Suppliers (Preliminary and Untested)

Model

Vendor

Dimensions (mm)

DC R(max)

DO3314-222MXC

Coilcraft

3.3 x 3.3 x 1.4

200 m

Ω

LPO3310-222MX

Coilcraft

3.3 x 3.3 x 1.0

150 m

Ω

ELL5GM2R2N

Panasonic

5.2 x 5.2 x x 1.5

53 m

Ω

CDRH2D142R2

Sumida

3.2 x 3.2 x 1.55

94 m

Ω

INPUT CAPACITOR SELECTION

A ceramic input capacitor of 10 µF is sufficient for most applications. A larger value may be used for improved

input voltage filtering. Use X7R or X5R type capacitors, do not use Y5V. The DC bias characteristics of ceramic

capacitors must be considered when selecting case sizes of 0805 or smaller for use in the application. Smaller

case sizes in many cases exhibit a large drop in capacitance value as the DC bias increases.

The input filter capacitor supplies current to the PFET switch of the converter in the first half of each cycle and

reduces voltage ripple imposed on the input power source. A ceramic capacitor’s low ESR provides the best

noise filtering of the input voltage spikes due to this rapidly changing current. Select an input filter capacitor with

a surge current rating sufficient for the power-up surge from the input power source. The power-up surge current

is approximately the capacitor’s value (µF) times the voltage rise rate (V/µs).

The input current ripple can be calculated as:

(5)

The worst case IRMS is:

(6)

OUTPUT CAPACITOR SELECTION

A 10 µF capacitor is recommended for use at the output of the buck converter. Use X7R or X5R type capacitors;

do not use Y5V. The DC bias characteristics of ceramic capacitors must be considered when selecting case

sizes of 0805 or smaller for use in the application. Smaller case sizes in many cases exhibit a large drop in

capacitance value as the DC bias increases. DC bias characteristics vary from manufacturer to manufacturer and

dc bias curves should be requested from them as part of the capacitor selection process.

Copyright © 2010–2013, Texas Instruments Incorporated

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