SBVS343 March 2019 TPS7A78
ADVANCE INFORMATION for pre-production products; subject to change without notice.
The use of the capacitive-dropper, or cap-drop is not new in the realm of low-power applications like e-meters. The cap-drop CS acts as a voltage dropper where its reactance limits the maximum AC current that is used to charge the bulk capacitor CSCIN. The max AC supply voltage that can be used is determined by the availability of high voltage CS capacitor for such applications. The size of the CS cap is determined by many factor related to the solution such as the AC line voltage, AC line frequency, load current, output voltage, and bridge configuration of the device, use Table 2 for proper selection of CS capacitor for various AC supply voltage, output voltage and load current.
Although oversizing CS capacitor is not desirable due to apparent-power limitation in some applications in addition to increase in solution size, but it is recommended for applications that require faster start-up time or a relatively heavy load currents for the whole solution. Slightly oversizing CS is also recommended to swamp out long term degradation for high voltage low power applications such as e-meters.
As CS size increases, the max AC charging current increases leading to a faster charging of the bulk capacitor CSCIN and maintaining the headroom between VLDO_IN and VLDO_OUT under heavy load conditions.