8.1.2 Input and Output Capacitors Requirements

All the capacitors illustrated in Figure 14 or Figure 15 are required for proper operation. The value of C_S required to support the application current is obtained from the Calculating the Cap-Drop Capacitor CS section. The chosen C_S capacitor must tolerate the peak V_AC supply voltage of the application and meet the required safety requirements.

Choosing an a larger value of the C_S capacitor than required has an adverse effect on the standby power consumption; however, capacitance reduction over long-term service is inevitable and must be considered when selecting the value of C_S. A ceramic capacitor can be used as C_S in designs for lower AC supply voltages, but the capacitor voltage rating must be appropriate to the application.

For switching capacitors C_SC1 and C_SC2, select the minimum-required capacitor values and voltage ratings specified in the Recommended Operating Conditions table. Using too large of a capacitor for the switching capacitors is not recommended because a large capacitor lengthens the start-up time and load transient recovery time of the entire solution. Keep the switching capacitors as close to the device as possible to eliminate any unwanted trace inductance.

For the bulk capacitor C_SCIN, use the minimum required capacitor value obtained from the Calculating the Bulk Capacitor CSCIN section and increase that value based on the expected capacitor degradation resulting from aging and operating conditions. Accounting for capacitor degradation is especially important if a relatively low life expectancy of the capacitor is expected when an electrolytic capacitor is used. If the application requires an extended hold-up time, the values of the C_SCIN or C_{LDO_IN} capacitors can be increased as long as the maximum capacitor values specified in the Recommended Operating Conditions table are not exceeded. Using a significantly larger values of C_SCIN or C_{LDO_IN} has an adverse effect on the startup time of the solution.

For the C_{LDO_OUT} capacitor, maintain a 10:1 ratio between C_{LDO_IN} and C_{LDO_OUT} for applications using the maximum load current. For lesser load currents, the minimum required C_{LDO_OUT} and C_{LDOU_IN} capacitors are sufficient. For optimum performance, place all capacitors as close as possible to the device.