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Output Capacitor Selection I RMSCout + Vout 1 * Vout Vin L ƒ 1 2 3 (3) DVout + Vout 1 * Vout Vin L ƒ 1 8 Cout ƒ ) ESR (4) Input Capacitor Selection SLVS799C – NOVEMBER 2007 – REVISED JANUARY 2009 ........................................................................................................................................... www.ti.com Table 2. List of Inductors DIMENSIONS INDUCTOR TYPE SUPPLIER [mm3] 2.5 × 2.0 × 1.0 MIPS2520 FDK 2.5 × 2.0 × 1.2 MIPSA2520 FDK 2.5 × 2.0 × 1.0 KSLI-252010AG2R2 Hitachi Metals 2.5 × 2.0 × 1.2 LQM2HPN2R2MJ0L Murata The advanced fast-response voltage mode control scheme of the TPS62270 allows the use of tiny ceramic capacitors. Ceramic capacitors with low ESR values have the lowest output voltage ripple and are recommended. The output capacitor requires either an X7R or X5R dielectric. Y5V and Z5U dielectric capacitors, aside from their wide variation in capacitance over temperature, become resistive at high frequencies. At nominal load current, the device operates in PWM mode and the RMS ripple current is calculated as: At nominal load current, the device operates in PWM mode and the overall output voltage ripple is the sum of the voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and discharging the output capacitor: At light load currents the converter operates in Power Save Mode and the output voltage ripple is dependent on the output capacitor and inductor value. Larger output capacitor and inductor values minimize the voltage ripple in PFM mode and tighten DC output accuracy in PFM mode. An input capacitor is required for best input voltage filtering, and minimizing the interference with other circuits caused by high input voltage spikes. For most applications, a 4.7 µF to 10µF ceramic capacitor is recommended. Because ceramic capacitor loses up to 80% of its initial capacitance at 5 V, it is recommended that 10 µF input capacitors be used for input voltages >4.5V. The input capacitor can be increased without any limit for better input voltage filtering. Take care when using only small ceramic input capacitors. When a ceramic capacitor is used at the input and the power is being supplied through long wires, such as from a wall adapter, a load step at the output or VIN step on the input can induce ringing at the VIN pin. This ringing can couple to the output and be mistaken as loop instability or could even damage the part by exceeding the maximum ratings. Table 3. List of Capacitors CAPACITANCE TYPE SIZE mm3 SUPPLIER 4.7 µF GRM188R60J475K 0603: 1.6 × 0.8 × 0.8 mm3 Murata 10 µF GRM188R60J106M69D 0603: 1.6 × 0.8 × 0.8 mm3 Murata 16 Copyright © 2007–2009, Texas Instruments Incorporated |
