LM2621

SNVS033C – MAY 2004 – REVISED MARCH 2005

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Design Procedure

SETTING THE OUTPUT VOLTAGE

The output voltage of the step-up regulator can be set between 1.24V and 14V by connecting a feedback

(1)

A value of 150k

the peak-to-peak output voltage ripple as well as improve the efficiency of the step-up regulator, because a set

hysteresis of 30mV at the FB pin is used for the gated oscillator control scheme.

BOOTSTRAPPING

stepped down using a linear regulator, or a different voltage source available in the system. This is referred to as

non-bootstrapped operation. The maximum acceptable voltage at the BOOT pin (Pin 7) is 10V.

SETTING THE SWITCHING FREQUENCY

frequency. A high switching frequency allows the use of very small surface mount inductors and capacitors and

results in a very small solution size. A switching frequency between 300kHz and 2MHz is recommended.

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.

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