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LM2621MMX Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM2621MMX Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 11 page Design Procedure SETTING THE OUTPUT VOLTAGE The output voltage of the step-up regulator can be set be- tween 1.24V and 14V by connecting a feedback resistive divider made of R F1 and RF2. The resistor values are se- lected as follows: R F2 =RF1 /[(VOUT/ 1.24) −1] A value of 150k Ω is suggested for R F1. Then, RF2 can be selected using the above equation. A 39pF 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 30mV at the FB pin is used for the gated oscillator control scheme. BOOTSTRAPPING When the output voltage (V OUT) is between 2.5V and 5.0V a bootstrapped operation is suggested. This is achieved by connecting the V DD pin (Pin 6) to VOUT. However if the VOUT is outside this range, the V DD pin should be connected to a voltage source whose range is between 2.5V and 5V. This can be the input voltage (V IN), VOUT 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 The switching frequency of the oscillator is selected by choosing an external resistor (R FQ) connected between FREQ and V DD pins. See the graph titled „Switching Fre- quency vs R FQ“ in the Typical Operating Characteristics section of the datasheet for choosing the R FQ value to achieve the desired switching frequency. A high switching frequency allows the use of very small surface mount induc- tors and capacitors and results in a very small solution size. A switching frequency between 300kHz and 2MHz is recom- mended. 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 effi- ciency. Open-core inductors cause flux linkage with circuit compo- nents 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 tan- talum 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 Tel: (800) 322-2645 Fax: (708) 639-1469 Sprague/ Vishay Tel: (207) 324-4140 Fax: (207) 324-7223 Motorola Tel: (800) 521-6274 Fax: (602) 244-6609 Coiltronics Tel: (407) 241-7876 Fax: (407) 241-9339 Kemet Tel: (864) 963-6300 Fax: (864) 963-6521 International Rectifier (IR) Tel: (310) 322-3331 Fax: (310) 322-3332 Pulse Engineering Tel: (619) 674-8100 Fax: (619) 674-8262 Nichicon Tel: (847) 843-7500 Fax: (847) 843-2798 General Semiconductor Tel: (516) 847-3222 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 short, direct and wide traces. The voltage feedback network (R F1,RF2, and CF1) should be kept very close to the FB pin. Noisy traces, such as from the SW pin, should be kept away from the FB and V DD pins. The traces that run between Vout and the FB pin of the IC should be kept away from the inductor flux. Always provide sufficient 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. www.national.com 8 |
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