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CXL180 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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CXL180 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page can be reduced by applying a correction factor to the feedback resistor equation. First, calculate the correction factor, k, by plugging the desired output voltage into the following formula: k represents the shift in the operating point at the feed- back node (OUT). Select the lower feedback resistor, R3, to be ≤35.7kΩ to ensure stability and solve for R2: Inductor Selection A 4.7µH inductor with a saturation current of at least 800mA is recommended for the MAX1557 full-load (600mA) application. For the MAX1556/MAX1556A appli- cation with 1.2A full load, use a 3.3µH inductor with at least 1.34A saturation current. For lower full-load cur- rents the inductor current rating can be reduced. For maximum efficiency, the inductor’s resistance (DCR) should be as low as possible. Please note that the core material differs among different manufacturers and inductor types and has an impact on the efficiency. See Table 2 for recommended inductors and manufacturers. Capacitor Selection Ceramic input and output capacitors are recommend- ed for most applications. For best stability over a wide temperature range, use capacitors with an X5R or bet- ter dielectric due to their small size, low ESR, and low temperature coefficients. Output Capacitor The output capacitor COUT is required to keep the out- put voltage ripple small and to ensure regulation loop stability. COUT must have low impedance at the switch- ing frequency. A 22µF ceramic output capacitor is rec- ommended for most applications. If a larger output capacitor is used, then paralleling smaller capacitors is suggested to keep the effective impedance of the capacitor low at the switching frequency. Input Capacitor Due to the pulsating nature of the input current in a buck converter, a low-ESR input capacitor at INP is required for input voltage filtering and to minimize interference with other circuits. The impedance of the input capacitor CINP should be kept very low at the switching frequen- cy. A minimum value of 10µF is recommended at INP for most applications. The input capacitor can be increased for better input filtering. IN Input Filter In all MAX1557 applications, connect INP directly to IN and bypass INP as described in the Input Capacitor section. No additional bypass capacitor is required at IN. For applications using the MAX1556 and MAX1556A, an RC filter between INP and IN keeps power-supply noise from entering the IC. Connect a 100 Ω resistor between INP and IN, and connect a 0.47µF capacitor from IN to GND. Soft-Start Capacitor The soft-start capacitor, CSS, is required for proper operation of the MAX1556/MAX1556A/MAX1557. The recommended value of CSS is discussed in the Soft- Start section. Soft-start times for various soft-start capacitors are shown in the Typical Operating Characteristics. PCB Layout and Routing Due to fast-switching waveforms and high-current paths, careful PCB layout is required. An evaluation kit (MAX1556EVKIT) is available to speed design. When laying out a board, minimize trace lengths between the IC, the inductor, the input capacitor, and the output capacitor. Keep these traces short, direct, and wide. Keep noisy traces, such as the LX node trace, away from OUT. The input bypass capacitors should be placed as close as possible to the IC. Connect GND to the exposed paddle and star PGND and GND together at the output capacitor. The ground connections of the input and output capacitors should be as close together as possible. 075 3 32 . Vk V R RR OUTPUT − ⎛ ⎝⎜ ⎞ ⎠⎟ = + () kx V x VV V OUTPUT . . . = − ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ − 106 10 075 36 2 16µA IQ, 1.2A PWM DC-DC Step-Down Converters 10 ______________________________________________________________________________________ OUTPUT OUT R2 R3 SS REFERENCE 1.25V ERROR AMPLIFIER Figure 6. Adjustable Output Voltage |
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