LM3671 datasheet(14/16 Pages) NSC

14 / 16 page

Application Information (Continued)

TABLE 1. 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

CDRH2D14-2R2

Sumida

3.2 x 3.2 x 1.55

94 m

Ω

OUTPUT CAPACITOR SELECTION

Use a 22 µF, 6.3V ceramic capacitor. Use X7R or X5R types,

do not use Y5V. DC bias characteristics of ceramic capaci-

tors 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 LM3671

has been evaluated with 22 µF, 6.3V, 0805 with worst case

tolerances including dc bias effects. The use of two 10 µF,

6.3V, 0805 caps will give an overall higher capacitance value

when dc bias is considered.

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 func-

tions.

The output voltage ripple is caused by the charging and

discharging of the output capacitor and also due to its R

ESR

and can be calculated as:

Voltage peak-to-peak ripple due to capacitance can be ex-

pressed as follows

Voltage peak-to-peak ripple due to ESR can be expressed

as follows

PP-ESR = (2*IRIPPLE) * RESR

Because these two components are out of phase the rms

value can be used to get an approximate value of peak-to-

peak ripple.

Voltage peak-to-peak ripple, root mean squared can be ex-

pressed as follows

Note that the output voltage ripple is dependent on the

inductor current ripple and the equivalent series resistance

of the output capacitor (R

ESR).

The R

ESR is frequency dependent (as well as temperature

dependent); make sure the value used for calculations is at

the switching frequency of the part.

TABLE 2. Suggested Capacitors and Their Suppliers

Model

Type

Vendor

Voltage Rating

Case Size

Inch (mm)

22 µF for C

OUT

GRM21BR60J226K

Ceramic, X5R

Murata

6.3V

0805 (2012)

C2012X5R0J226K

Ceramic, X5R

TDK

6.3V

0805 (2012)

JMK212BJ226K

Ceramic, X5R

Taiyo-Yuden

6.3V

0805 (2012)

10 µF for C

GRM21BR60J106K

Ceramic, X5R

Murata

6.3V

0805 (2012)

JMK212BJ106K

Ceramic, X5R

Taiyo-Yuden

6.3V

0805 (2012)

C2012X5R0J106K

Ceramic, X5R

TDK

6.3V

0805 (2012)

BOARD LAYOUT CONSIDERATIONS

PC board layout is an important part of DC-DC converter

design. Poor board layout can disrupt the performance of a

DC-DC converter and surrounding circuitry by contributing to

EMI, ground bounce, and resistive voltage loss in the traces.

These can send erroneous signals to the DC-DC converter

IC, resulting in poor regulation or instability.

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