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MAX1920EUT Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX1920EUT Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 11 page MAX1921 Using Ceramic COUT When using the application circuit of Figure 1, the inductor’s series resistance causes a small amount of load regulation, as desired for a voltage-positioning load transient response. Choose R1 such that VOUT is high at no load by about half of this load regulation. The simplified calculation is: where RL(MAX) is the maximum series resistance of the inductor. Select a standard resistor value that is within 20% of this calculation. Next, calculate CFF for 25mV ripple at the internal feed- back node. The simplified calculation is: where R1 is the standard resistor value that is used. Select a standard capacitor value that is within 20% of the calculated CFF. MAX1920 Using Ceramic COUT When using the application circuit of Figure 2, the induc- tor’s series resistance causes a small amount of load regulation, as desired for a voltage-positioning load tran- sient response. Choose R1 and R2 such that VOUT is high at no load by about half of this load regulation: where R2 is chosen in the 50k Ω to 500kΩ range, VREF = 1.25V and RL is the typical series resistance of the inductor. Use 1% or better resistors. Next, calculate the equivalent resistance at the FB node as: Then, calculate CFF for 25mV ripple at FB. The simpli- fied calculation is: Select a standard capacitor value that is within 20% of the calculated CFF. MAX1920 Using Tantalum COUT When using the application circuit of Figure 4, choose R1 and R2 such as to obtain the desired VOUT: where R2 is chosen to be less than 50k Ω and VREF = 1.25V. Use 1% or better resistors. Layout Considerations 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 inductor, input filter capacitor, and output filter capacitor as close to the device as possible, and keep their traces short, direct, and wide. Connect their ground pins at a single common node in a star ground configuration. The external voltage-feedback network should be very close to the FB pin, within 0.2in (5mm). Keep noisy traces, such as the LX trace, away from the voltage- feedback network; also keep them separate, using grounded copper. The MAX1920/MAX1921 evaluation kit data sheet includes a proper PC board layout and routing scheme. RR V V OUT REF 12 1 =× − Cq FF =× − 25 10 5 . Re Re || qR R RR RR == × + 12 12 12 RR VR I MAX V OUT L OUT REF 12 2 1 =× +× − () / CR FF =× − 25 10 1 5 . RR MAX L 15 10 4 =× × () Low-Voltage, 400mA Step-Down DC-DC Converters in SOT23 8 _______________________________________________________________________________________ Table 3. Component Suppliers SUPPLIER PHONE WEBSITE Coilcraft 847-639-6400 www.coilcraft.com Kemet 408-986-0424 www.kemet.com Murata 814-237-1431 www.murata.com USA 847-956-0666 Sumida Japan 81-3-3607-5111 www.sumida.com USA 408-573-4150 www.T-Yuden.com Taiyo Yuden Japan 81-3-3833-5441 www.yuden.co.jp USA 847-297-0070 www.tokoam.com Toko Japan 81-3-3727-1161 www.toko.co.jp MAX1920 IN AGND SHDN LX PGND FB OFF ON R1 COUT L OUTPUT UP TO 400mA INPUT 2V TO 5.5V CIN R2 2 1 3 5 6 4 Figure 4. MAX1920 Application Circuit Using Tantalum Output Capacitor |
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