SBVS343 March 2019 TPS7A78
ADVANCE INFORMATION for pre-production products; subject to change without notice.
The value of the minimum surge resistor RS (min) is determined by the peak AC supply voltage of the application. The value can be calculated using Equation 3, where VAC_MAX is the peak AC supply voltage for the application, and ISurge is the max AC transient current that TPS7A78 pin AC- or AC+ pins can tolerate as specified in the Recommended Operating Conditions table.
The Maximum AC supply voltage is bound by the availability of high voltage cap-drop capacitor CS. As for surge resistors RS value shall be << 1/(2πf×CS). If RS value is comparable or higher than the reactance of CS, then the max output current will be limited by both components rather than just by CS.
If the solution requires the use of Transient Voltage Surge Suppressor (TVS) or a Metal-Oxide Varistors (MOV), then the minimum value of the surge resistor RS (min) can be calculated using the max clamping voltage of the TVS/MOV which is used for the solution. Table 2 provides the values for RS (min) resistor for a standard AC supply, use Equation 3 should your application has a different max AC supply voltage.