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MAX1878 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX1878 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page Inductor Selection The MAX1878 is optimized to use a 10µH inductor over the entire operating range. Smaller inductance values typically offer smaller physical size for a given series resistance or saturation current. Circuits using larger inductance values may startup at lower input voltages and exhibit less ripple, but also provide reduced output power. This occurs when the inductance is sufficiently large to prevent the maximum current limit from being reached before the maximum on-time expires. The inductor’s saturation current rating should be greater than the peak switching current. However, it is generally acceptable to bias the inductor into saturation by as much as 20%, although this will slightly reduce efficien- cy. Choose a low DC-resistance inductor to improve effi- ciency. For the above reasons choose the step-up converter inductor in the range of 10µH to 33µH depending on the input voltage (4µH per volt of VIN). Step-Up Converter Diode Selection The high maximum switching frequency of 500kHz requires a high-speed rectifier such as the 1N4148. To maintain high efficiency, the average current rating of the diode should be greater than the peak switching current. Choose a reverse breakdown voltage greater than the output voltage. A Schottky diode is not recom- mended as the lower forward voltage does little to improve efficiency whereas the higher reverse leakage current decreases efficiency. Input Bypass Capacitors Bypass VIN with a 10µF low-ESR surface-mount ceram- ic capacitor to PGND and PGNDLCD as close to the IC as possible. This input bypass capacitor reduces peak currents and noise at the input voltage source. Connect AIN1 and AIN2 together and bypass with a low-ESR 1µF surface-mount ceramic capacitor to AGND. A low resistance (10Ω) from IN to AIN1 and AIN2 creates a lowpass RC filter and provides low-noise analog input power to the MAX1878. Output Filter Capacitors The MAX1878 is a voltage mode converter and requires ripple at FB and FBLCD for stable regulation. For most applications, bypass VLCD with a 0.1µF small ceramic surface-mount capacitor to PGNDLCD. For small ceramic capacitors, the output ripple voltage is domi- nated by the capacitance value. If tantalum or electrolyt- ic capacitors are used, the higher ESR increases the output ripple voltage. Decreasing the ESR reduces the output ripple voltage and the peak-to-peak transient voltage. Surface-mount capacitors are generally pre- ferred because they lack the inductance and resistance of their through-hole equivalents. Bypass VMAIN with a 10µF to 47µF tantalum capacitor to PGND. Choose a capacitor with 200mΩ to 300mΩ ESR to provide stable switching while minimizing output ripple. A 22µF filter capacitor works well for most applications. Ripple Regulation For proper switching control the ripple at FB and FBLCD must be greater than 25mV. Use R6 and C6 as shown in Figure 1 to inject ripple into FB. To insure sufficient ripple on FBLCD, connect C5 as shown in Figure 1. PC Board Layout and Grounding High switching frequencies make PC board layout a very important part of design. Good design minimizes excessive EMI on the feedback paths and voltage gra- dients in the ground plane, both of which can result in instability or regulation errors. Connect the inductors, input filter capacitors, and output filter capacitors as close to the device as possible, and keep their traces short, direct, and wide. The external voltage-feedback networks should be very close to the feedback pins, within 0.2 inches (5mm). Keep noisy traces, such as LX and LXLCD, away from the voltage feedback networks; also keep them separate, using grounded copper. The exposed backside pad and corner tabs of the TQFN package are internally connected to analog ground. For heat dissipation, connect the exposed backside pad to a large analog ground plane, prefer- ably on a surface of the board that receives good air- flow. Connect all power grounds and all analog grounds to separate ground planes in a star ground configuration. Connect the analog ground plane and the power ground plane together at a single point. The MAX1878 evaluation kit data sheet includes a proper PC board layout and routing scheme. Dual-Output Step-Down and LCD Step-Up Power Supply for PDAs 10 ______________________________________________________________________________________ |
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