8.2.2.4 Calculating the Bulk Capacitor C_SCIN

The TPS7A78 uses a bulk capacitor C_SCIN to smooth the rectified DC voltage ripple on the SCIN pin and to supply charge to the switched capacitor stage; see the 4:1 Switched-Capacitor Voltage Reduction section. The C_SCIN capacitor also functions as a charge reservoir to hold-up the device output voltage for a period of time if the supply collapses. The minimum value of the C_SCIN capacitor required can be calculated using Equation 17 through Equation 20, however these equations make the following assumptions to simplify the C_SCIN capacitor calculation:

• The AC supply frequency is within ±5% of the nominal standard frequencies of 50 Hz and 60 Hz
• The voltage ripple on the SCIN pin is around from 0.5 V to 0.8 V.
• The AC impedance of the cap-drop capacitor C_S is at least ten times lower than that of the bulk capacitor C_SCIN and the surge resistor R_S

Use Equation 17 for the FB 60-Hz V_AC supply and Equation 18 for the HB 60-Hz V_AC supply.

Use Equation 19 for the FB 50-Hz V_AC supply and Equation 20 for the HB 50-Hz V_AC supply.

The calculated C_SCIN capacitor from Equation 17 through Equation 20 represents the minimum value required for the application example. However, Equation 17 through Equation 20 do not account for capacitance derating for all operating conditions. Follow the manufacturer recommendation and guidelines to ensure the minimum required capacitance needed for the application. See the Input and Output Capacitors Requirements section for more details.