LM3671 datasheet(13/16 Pages) NSC

13 / 16 page

Application Information (Continued)

LM3671-ADJ configurations for various V

OUT

OUT(V)

R1(k

Ω)R2 (kΩ)

C1 (pF)

C2 (pF)

L (µH)

IN (µF)

OUT(µF)

0.7

54.9

137

2.2

0.8

118

196

none

2.2

0.9

162

200

none

2.2

1.0

200

none

2.2

1.1

191

158

none

2.2

1.2

280

200

none

2.2

1.5

360

180

none

2.2

1.6

442

200

8.2

none

2.2

1.7

432

180

8.2

none

2.2

1.875

523

191

6.8

none

2.2

2.5

402

100

8.2

none

2.2

3.3

562

100

6.8

2.2

INDUCTOR SELECTION

There are two main considerations when choosing an induc-

tor; the inductor should not saturate, and the inductor current

ripple is small enough to achieve the desired output voltage

ripple. Different saturation current rating specs are followed

by different manufacturers so attention must be given to

details. Saturation current ratings are typically specified at

25˚C so ratings at max ambient temperature of application

should be requested from manufacturer.

There are two methods to choose the inductor saturation

current rating.

Method 1:

The saturation current is greater than the sum of the maxi-

mum load current and the worst case average to peak

inductor current. This can be written as

•

RIPPLE: average to peak inductor current

•

OUTMAX: maximum load current (600mA)

•

IN: maximum input voltage in application

•

L : min inductor value including worst case tolerances

(30% drop can be considered for method 1)

•

f : minimum switching frequency (1.6Mhz)

•

OUT: output voltage

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 1150mA.

A 2.2 µH inductor with a saturation current rating of at least

1150 mA is recommended for most applications.The induc-

tor’s resistance should be less than 0.3

Ω for good efficiency.

Table 1 lists suggested inductors and suppliers. For low-cost

applications, an unshielded bobbin inductor could be consid-

ered. 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 shielded

inductor, in the event that noise from low-cost bobbin models

is unacceptable.

INPUT CAPACITOR SELECTION

A ceramic input capacitor of 10 µF, 6.3V is sufficient for most

applications. Place the input capacitor as close as possible

to the V

IN pin of the device. A larger value may be used for

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 input filter capacitor supplies current to the PFET

switch of the LM3671 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:

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