TPS7A78: Rs/Cs selection

Hi Muk, 

The value of Cs is calculated based off the min VAC voltage and minimum line frequency, load current, and device bridge configuration. The selected high voltage capacitor must be able to tolerate the peak VAC voltage of the application, also capacitance loss under long term service is inevitable and should be considered in the design, follow the vender recommendation and guidelines for capacitor degradation.

As for Rs value; it is calculated based off the peak VAC voltage of VAC supply or based off the maximum clamping voltage of the TVS or MOV devices of the application.

Use the following equation for calculating Cs values then double the calculated value for your HB application then approximate up the doubled value to the nearest standard capacitor value.  

Cs = IOUT/ (16*f*(sqrt(2)* VAC(min) - 4*(VLDO_OUT(nom) + 0.6)))

where:

IOUT is the application load current, however, the
application peak current should be considered if it cannot be supported by the LDO output
capacitor.

ƒ is the minimum VAC supply frequency 

VLDO_OUT is the targeted  LDO output voltage. 

Use the following equation for calculating Rs, then approximate up the calculated value to the nearest standard value.

Rs = VAC(peak) / 2.5 A 

or

Rs = VAC (maximum clamping voltage ) / 2.5 A ( if the application uses TVS or MOV protection devices).

where:

2.5 A current is the peak transient current into/out of either AC+, or AC– pins (during hot plug for period of ~100 μs)

for your VAC supply and since you didn't specify any TVS or MOV devices are being used, Rs = (sqrt(2) * 276)/2.5 = 156.3 Ω  

Approximate up  the calculated Rs value to the nearest standard value. 

TI recommends limiting the IOUT current to 60 mA for HB configuration.

All of the above equations and explanations will be available in the full TPS7A78 device datasheet release in the upcoming few months. 

BR,

Haidar