LM2621

SNVS033C – MAY 2004 – REVISED MARCH 2005

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INDUCTOR SELECTION

The LM2621's high switching frequency enables the use of a small surface mount inductor. A 6.8µH shielded

inductor is suggested. The inductor should have a saturation current rating higher than the peak current it will

experience during circuit operation (see graph titled „Peak Inductor Current vs. Load Current” in the Typical

Performance Characteristics section). Less than 100m

Ω ESR is suggested for high efficiency.

Open-core inductors cause flux linkage with circuit components and interfere with the normal operation of the

circuit. They should be avoided. For high efficiency, choose an inductor with a high frequency core material, such

as ferrite, to reduce the core losses. To minimize radiated noise, use a toroid, pot core or shielded core inductor.

The inductor should be connected to the SW pin as close to the IC as possible. See Table 1 for a list of the

inductor manufacturers.

OUTPUT DIODE SELECTION

A Schottky diode should be used for the output diode. The forward current rating of the diode should be higher

than the load current, and the reverse voltage rating must be higher than the output voltage. Do not use ordinary

rectifier diodes, since slow switching speeds and long recovery times cause the efficiency and the load regulation

to suffer. Table 1 shows a list of the diode manufacturers.

INPUT AND OUTPUT FILTER CAPACITORS SELECTION

Tantalum chip capacitors are recommended for the input and output filter capacitors. A 22µF capacitor is

suggested for the input filter capacitor. It should have a DC working voltage rating higher than the maximum

input voltage. A 68µF tantalum capacitor is suggested for the output capacitor. The DC working voltage rating

should be greater than the output voltage. Very high ESR values (>3

Ω) should be avoided. Table 1 shows a list

of the capacitor manufacturers.

Table 1. Suggested Manufacturers List

Inductors

Capacitors

Diodes

Coilcraft

Sprague/ Vishay

Motorola

Tel: (800) 322-2645

Tel: (207) 324-4140

Tel: (800) 521-6274

Fax: (708) 639-1469

Fax: (207) 324-7223

Fax: (602) 244-6609

Coiltronics

Kemet

International Rectifier (IR)

Tel: (407) 241-7876

Tel: (864) 963-6300

Tel: (310) 322-3331

Fax: (407) 241-9339

Fax: (864) 963-6521

Fax: (310) 322-3332

Pulse Engineering

Nichicon

General Semiconductor

Tel: (619) 674-8100

Tel: (847) 843-7500

Tel: (516) 847-3222

Fax: (619) 674-8262

Fax: (847) 843-2798

Fax: (516) 847-3150

PC BOARD LAYOUT

High switching frequencies and high peak currents make a proper layout of the PC board an important part of

design. Poor design can cause excessive EMI and ground-bounce, both of which can cause malfunction and loss

of regulation by corrupting voltage feedback signal and injecting noise into the control section.

Power components - such as the inductor, input and output filter capacitors, and output diode - should be placed

as close to the regulator IC as possible, and their traces should be kept short, direct and wide. The ground pins

of the input and output filter capacitors and the PGND and SGND pins of LM2621 should be connected using

copper area to dissipate the heat due to power loss in the circuitry and prevent the thermal protection circuitry in

the IC from shutting the IC down.

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