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LM2621 Datasheet(PDF) 8 Page - Texas Instruments

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Part No. LM2621
Description  LM2621 Low Input Voltage, Step-Up DC-DC Converter
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI - Texas Instruments

LM2621 Datasheet(HTML) 8 Page - Texas Instruments

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LM2621
SNVS033C – MAY 2004 – REVISED MARCH 2005
www.ti.com
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
resistive divider made of RF1 and RF2. The resistor values are selected as follows:
RF2 = RF1 /[(VOUT/ 1.24) −1]
(1)
A value of 150k
Ω is suggested for RF1. Then, RF2 can be selected using the above equation. A 39pF capacitor
(CF1) connected across RF1 helps in feeding back most of the AC ripple at VOUT 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 (VOUT) is between 2.5V and 5.0V a bootstrapped operation is suggested. This is
achieved by connecting the VDD pin (Pin 6) to VOUT. However if the VOUT is outside this range, the VDD pin should
be connected to a voltage source whose range is between 2.5V and 5V. This can be the input voltage (VIN), 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 (RFQ) connected between
FREQ and VDD pins. See the graph titled " Switching Frequency vs RFQ” in the TYPICAL PERFORMANCE
CHARACTERISTICS section of the datasheet for choosing the RFQ 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 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.
8
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Copyright © 2004–2005, Texas Instruments Incorporated
Product Folder Links: LM2621


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