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CS51413 Arkusz danych(PDF) 11 Page - ON Semiconductor |
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CS51413 Arkusz danych(HTML) 11 Page - ON Semiconductor |
11 / 16 page CS51411, CS51412, CS51413, CS51414 http://onsemi.com 11 The total output ripple is the sum of the VRIPPLE(ESR) and VRIPPLE(ESR). Figure 12. The Output Voltage Ripple Using Two 10 mF Ceramic Capacitors in Parallel Figure 13. The Output Voltage Ripple Using One 100 mF POSCAP Capacitor Figure 14. The Output Voltage Ripple Using One 100 mF OS–CON Figure 15. The Output Voltage Ripple Using One 100 mF Tantalum Capacitor Figure 12 to Figure 15 show the output ripple of a 5.0 V to 3.3 V/500 mA regulator using 22 µH inductor and various capacitor types. At the switching frequency, the low ESR and ESL make the ceramic capacitors behave capacitively as shown in Figure 12. Additional paralleled ceramic capacitors will further reduce the ripple voltage, but inevitably increase the cost. “POSCAP”, manufactured by SANYO, is a solid electrolytic capacitor. The anode is sintered tantalum and the cathode is a highly conductive polymerized organic semiconductor. TPC series, featuring low ESR and low profile, is used in the measurement of Figure 13. It is shown that POSCAP presents a good balance of capacitance and ESR, compared with a ceramic capacitor. In this application, the low ESR generates less than 5.0 mV of ripple and the ESL is almost unnoticeable. The ESL of the through–hole OS–CON capacitor give rise to the inductive impedance. It is evident from Figure 14 which shows the step rise of the output ripple on the switch turn–on and large spike on the switch turn–off. The ESL prevents the output capacitor from quickly charging up the parasitic capacitor of the inductor when the switch node is pulled below ground through the catch diode conduction. This results in the spike associated with the falling edge of the switch node. The D package tantalum capacitor used in Figure 15 has the same footprint as the POSCAP, but doubles the height. The ESR of the tantalum capacitor is apparently higher than the POSCAP. The electrolytic and tantalum capacitors provide a low–cost solution with compromised performance. The reliability of the tantalum capacitor is not a serious concern for output filtering because the output capacitor is usually free of surge current and voltage. Diode Selection The diode in the buck converter provides the inductor current path when the power switch turns off. The peak reverse voltage is equal to the maximum input voltage. The peak conducting current is clamped by the current limit of the IC. The average current can be calculated from: |
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