LM3671_08 Datasheet, PDF(19/24 Page) National Semiconductor (TI)

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LM3671_08 Datasheet, PDF (19/24 Pages) National Semiconductor (TI) – 2MHz, 600mA Step-Down DC-DC Converter

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TABLE 1. Suggested Inductors and Their Suppliers

Model

DO3314-222MX

LPO3310-222MX

ELL5GM2R2N

CDRH2D14NP-2R2NC

Vendor

Coilcraft

Panasonic

Sumida

Dimensions LxWxH(mm)

3.3 x 3.3 x 1.4

3.3 x 3.3 x 1.0

5.2 x 5.2 x 1.5

3.2 x 3.2 x 1.55

D.C.R (max)

200 mâ¦

150 mâ¦

53 mâ¦

94 mâ¦

OUTPUT CAPACITOR SELECTION

A ceramic output capacitor of 10 ÂµF, 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 char-

acteristics vary from manufacturer to manufacturer and dc

bias curves should be requested from them as part of the ca-

pacitor selection process.

The minimum output capacitance to guarantee good per-

formance is 5.75ÂµF at 1.8V dc bias including tolerances

and over ambient temperature range. The output filter ca-

pacitor 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 ca-

pacitors must be selected with sufficient capacitance and

sufficiently low ESR to perform these functions.

The output voltage ripple is caused by the charging and dis-

charging of the output capacitor and by the RESR and can be

calculated as:

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

pressed as follow:

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

follow:

VPP-ESR = (2 * IRIPPLE) * RESR

Because these two components are out of phase the rms (root

mean squared) value can be used to get an approximate val-

ue of peak-to-peak ripple.

The peak-to-peak ripple voltage, rms value can be expressed

as follow:

Note that the output voltage ripple is dependent on the induc-

tor current ripple and the equivalent series resistance of the

output capacitor (RESR).

The RESR 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

4.7 ÂµF for CIN

C2012X5R0J475K

JMK212BJ475K

GRM21BR60J475K

C1608X5R0J475K

10 ÂµF for COUT

GRM21BR60J106K

JMK212BJ106K

C2012X5R0J106K

C1608X5R0J106K

Type

Ceramic, X5R

Vendor

TDK

Taiyo-Yuden

Murata

TDK

Murata

Taiyo-Yuden

TDK

Voltage Rating

6.3V

Case Size

Inch (mm)

0805 (2012)

0603 (1608)

0805 (2012)

0603 (1608)

Micro SMD PACKAGE ASSEMBLY AND USE

Use of the Micro SMD package requires specialized board

layout, precision mounting and careful re-flow techniques, as

detailed in National Semiconductor Application Note 1112.

Refer to the section "Surface Mount Technology (SMD) As-

sembly Considerations". For best results in assembly, align-

ment ordinals on the PC board should be used to facilitate

placement of the device. The pad style used with Micro SMD

package must be the NSMD (non-solder mask defined) type.

This means that the solder-mask opening is larger than the

pad size. This prevents a lip that otherwise forms if the solder-

mask and pad overlap, from holding the device off the surface

of the board and interfering with mounting. See Application

Note 1112 for specific instructions how to do this. The 5-Bump

package used for LM3671 has 300 micron solder balls and

requires 10.82 mils pads for mounting on the circuit board.

The trace to each pad should enter the pad with a 90Â° entry

angle to prevent debris from being caught in deep corners.

Initially, the trace to each pad should be 7 mil wide, for a sec-

tion approximately 7 mil long or longer, as a thermal relief.

Then each trace should neck up or down to its optimal width.

The important criteria is symmetry. This ensures the solder

bumps on the LM3671 re-flow evenly and that the device sol-

ders level to the board. In particular, special attention must be

paid to the pads for bumps A1 and A3, because VIN and GND

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