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LX1910 Datasheet(PDF) 5 Page - Microsemi Corporation |
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LX1910 Datasheet(HTML) 5 Page - Microsemi Corporation |
5 / 8 page Microsemi Integrated Products Division 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 5 Copyright © 2004 Rev. 1.1, 2005-02-25 LX1910 Step-Down Regulator PRODUCTION DATA SHEET TM ® APPLI CATI ON NOTE FUNCTIONAL DESCRIPTION The LX1910 implements a PFM / PWM architecture that improves power management efficiency across the output load range. OUTPUT VOLTAGE PROGRAMMING Resistors R1 and R2 program the output voltage. An optional capacitor CX may be inserted across R1 to improve the transient response (see Figure 1). The value of R2 should be less than 100KΩ. The value of R1 can be determined using the following equation, note VREF is also referred to as VFBT. 1 V V R2 R1 REF OUT ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ − ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = DESIGN EXAMPLE: Let R2 equal 50K and the required VOUT equal to 3.0V. 78KΩ 1 50K R1 1.17 3V = − = ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ DIODE SELECTION A Schottky diode is recommended for use with the LX1910 because it provides fast switching and superior reverse recovery performance. The Microsemi UPS5817 (20V @ 1A) makes an effective choice for most applications. INDUCTOR SELECTION Selecting the appropriate inductor type and value ensures optimal performance of the converter circuit for the intended application. This selection process requires the designer to make trade-offs between circuit performance and cost. A primary consideration requires the selection of an inductor that will not saturate at the peak current level. Other considerations that affect inductor choice include EMI, output voltage ripple, and overall circuit efficiency. The inductor that works best depends upon the application’s requirements. Further, some experimentation with actual devices in-circuit is typically necessary to make the most effective choice. The LX1910 allows for a broad selection of inductor values and choosing a value between 2.2µH and 30µH supports a majority of applications. Selecting a larger inductor value can increase efficiency and reduce output voltage ripple. Smaller inductors typically provide smaller package size (critical in many portable applications) at the expense of increasing output ripple current. Regardless of inductor value, selecting a device manufactured with a ferrite-core produces lower losses at higher switching frequencies and thus better overall performance. CAPACITOR SELECTION To minimize ripple voltage, output capacitors with a low series resistance (ESR) are recommended. Multi-layer ceramic capacitors with X5R or X7R dielectric make an effective choice because they feature small size, very low ESR, a temperature stable dielectric, and can be connected in parallel to increase capacitance. Typical capacitance values of 4.7 to 30µF have proven effective. Other low ESR capacitors such as solid tantalum, specialty polymer, or organic semiconductor, make effective choices provided that the capacitor is properly rated for the output voltage and ripple current. Finally, choose an input capacitor of sufficient size to effectively decouple the input voltage source impedance (e.g., CIN > 4.7µF). LAYOUT CONSIDERATIONS The high peak currents and switching frequencies present in DC/DC converter applications require careful attention to device layout for optimal performance. Basic design rules include: (1) maintaining wide traces for power components (e.g., width > 50mils); (2) place CIN, COUT, the Schottky diode, and the inductor close to the LX1910; (3) minimizing trace capacitance by reducing the etch area connecting the SW pin to the inductor; and (4) minimizing the etch length to the FB pin to reduce noise coupling into this high impedance sense input. Other considerations include placing a 0.1uF capacitor between the LX1910 VOUT pin and GND pin to reduce high frequency noise and decoupling the VIN pin using a 0.1µF capacitor. |
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Similar Description - LX1910 |
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