8.2.1.2.1 Setting the Output Voltage

The output voltage of the step-up regulator can be set between 1.24 V and 14 V by connecting a feedback resistive divider made of R_F1 and R_F2. The resistor values are selected as follows:

A value of 150 kΩ is suggested for R_F1. Then, R_F2 can be selected using the above equation. A 39-pF capacitor (C_F1) connected across R_F1 helps in feeding back most of the AC ripple at V_OUT to the FB pin. This helps reduce the peak-to-peak output voltage ripple as well as improve the efficiency of the step-up regulator, because a set hysteresis of 30 mV at the FB pin is used for the gated oscillator control scheme.

8.2.1.2.2 Bootstrapping

When the output voltage (V_OUT) is between 2.5 V and 5.0 V a bootstrapped operation is suggested. This is achieved by connecting the V_DD pin (Pin 6) to V_OUT. However if the V_OUT is outside this range, the V_DD pin should be connected to a voltage source whose range is between 2.5 V and 5 V. This can be the input voltage (V_IN), V_OUT 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 10 V.

8.2.1.2.3 Setting the Switching Frequency

The switching frequency of the oscillator is selected by choosing an external resistor (R_FQ) connected between FREQ and V_DD pins. See Figure 9 for choosing the R_FQ value to achieve the desired switching 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 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-mΩ 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 (> 3Ω) should be avoided.