LM2621

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SNVS033D – MAY 2004 – REVISED NOVEMBER 2015

Typical Applications (continued)

8.2.1.2.2

Bootstrapping

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

8.2.1.2.3

Setting the Switching Frequency

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 300 kHz and 2 MHz is recommended.

8.2.1.2.4

Inductor Selection

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

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

experience during circuit operation (see Figure 10). Less than 100-m

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.

8.2.1.2.5

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.

8.2.1.2.6

Input and Output Filter Capacitors Selection

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

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

input voltage. A 68-µF 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.

8.2.1.3 Application Curves

Figure 15. Startup Vin=3.3V,Vout-5V, 10ms/div 1V/div

Figure 14. Startup Vin=1.2V,Vout-5V, 10ms/div 1V/div

(Ch3:Vin,Ch1:Vout)

(Ch3:Vin,Ch1:Vout)

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