I

RMS

OUTMAX

x

x (1 -

V

OUT

V

IN

V

OUT

V

IN

= I

=

r

)

+

12

2

The worst case is when

OUT

V

V

IN

(

)

OUT

V

V

IN

r

-

x

L

f

x

x

x

V

2 x

OUT

V

OUTMAX

I

where

2 x L

1

·

©

§

¹

·

©

§

¹

·

©

§

LM3674

www.ti.com

SNVS405F – DECEMBER 2005 – REVISED MAY 2013

(9)

•

•

•

•

L: min inductor value including worst case tolerances (30% drop can be considered for method 1)

•

f: minimum switching frequency (1.6 MHz)

•

Method 2:

A more conservative and recommended approach is to choose an inductor that has saturation current rating

greater than the max current limit of 1200 mA.

A 2.2 µH inductor with a saturation current rating of at least 1200 mA is recommended for most applications. The

inductor’s resistance should be less than around 0.3

Ω for good efficiency. Table 2 lists suggested 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 2. Suggested Inductors and Their Suppliers

Model

Vendor

Dimensions LxWxH (mm)

D.C.R (max)

DO3314-222MX

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 1.5

53 m

Ω

CDRH2D14NP-2R2NC

Sumida

3.2 x 3.2 x 1.55

94 m

Ω

INPUT CAPACITOR SELECTION

A ceramic input capacitor of 4.7 µF, 6.3V is sufficient for most applications. Place the input capacitor as close as

X5R types; do not use Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting

case sizes like 0805 and 0603. The minimum input capacitance to ensure good performance is 2.2µF at 3V

dc bias; 1.5µF at 5V dc bias including tolerances and over ambient temperature range. The input filter

capacitor supplies current to the PFET switch of the LM3674 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 a capacitor with sufficient ripple current rating.

The input current ripple can be calculated as:

(10)

OUTPUT CAPACITOR SELECTION

A ceramic output capacitor of 10 µF, 6.3V is sufficient for most applications. Use X7R or X5R types; do not use

Y5V. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and

0603. DC bias characteristics vary from manufacturer to manufacturer and dc bias curves should be requested

from them as part of the capacitor selection process.

The minimum output capacitance to ensure good performance is 5.75µF at 1.8V dc bias including

tolerances and over ambient temperature range. The output filter capacitor smoothes out current flow from

the inductor to the load, helps maintain a steady output voltage during transient load changes and reduces

output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low ESR to

perform these functions.

Copyright © 2005–2013, Texas Instruments Incorporated

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