•

I

•

I

•

V

•

L: min inductor value including worst case tolerances

(30% drop can be considered for method 1)

•

f: minimum switching frequency (1.6 MHz)

•

V

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 induc-

tor’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.

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 possible

to the V

improved input voltage filtering. 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. The minimum input capacitance to guarantee

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 rat-

ing. The input current ripple can be calculated as:

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

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