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# TPS7A78: The value of applied voltage to Csin

Part Number: TPS7A78

Hi

Could you please let me know the value of applied voltage to Cscin. (I know the recommended voltage rating is over 35V. I want to know the actual voltage value.)

Customer plan to use the Half-bridge configuration.

In this case,  There are Cs, Rs, diode and Cscin in series between ACin and GND.

Customer think, the applied voltage to Cscin is depend on the value of capacitance of Cscin and Cs.

In case of Cscin > Cs, Customer have the concern of higher voltage input.

Please let me know the calculation method for Cscin voltage.

Do you have any calculation tools?

Muk

• Hi Muk,

I understood that the customer is using TPS7A78 device in HB AC supply mode. If my understanding is correct then, the voltage on SCIN pin is going to be controlled (clamped) by the device Active Bridge control (clamp) as explained in section 7.3.1 of the datasheet and shall not be driven externally.

As for the 35V voltage rating the datasheet species, it is for CSCIN capacitor to account for capacitor derating under biasing voltage.

CSCIN capacitor value is always going to be greater than CS capacitor value to support certain load current as the datasheet specifies the values of CSCIN and CS in section 8.1.4 where equations 4 - 7 are used to calculated CSCIN capacitor value and table 2 for determining CS capacitor value.

As for a simple equation to estimate the voltage on SCIN pin and thus on CSCIN capacitor, you can use;

VSCIN = 4 (VLDO_OUT (nom) + 0.6 V)

So if the customer is using a 3.3V device, then VSCIN  = 4 (3.3 + 0.6) = 15.6 V.

We are planning on releasing  a calculator tool for the TPS7A78 external components as we are in the process of fully releasing the TPS7A78 device and is full detailed datasheet in the next few months.

Hope this helps for the moment and please could you share your customer use case and schematic (if you perfer to this offline) since HB configuration does not require a diode to be connected in series with CS and RS components as they are connected to either AC+ or AC- pins.

BR,

Haidar

• Hi Muk,

I understood that the customer is using TPS7A78 device in HB AC supply mode. If my understanding is correct then, the voltage on SCIN pin is going to be controlled (clamped) by the device Active Bridge control (clamp) as explained in section 7.3.1 of the datasheet and shall not be driven externally.

As for the 35V voltage rating the datasheet species, it is for CSCIN capacitor to account for capacitor derating under biasing voltage.

CSCIN capacitor value is always going to be greater than CS capacitor value to support certain load current as the datasheet specifies the values of CSCIN and CS in section 8.1.4 where equations 4 - 7 are used to calculated CSCIN capacitor value and table 2 for determining CS capacitor value.

As for a simple equation to estimate the voltage on SCIN pin and thus on CSCIN capacitor, you can use;

VSCIN = 4 (VLDO_OUT (nom) + 0.6 V)

So if the customer is using a 3.3V device, then VSCIN  = 4 (3.3 + 0.6) = 15.6 V.

We are planning on releasing  a calculator tool for the TPS7A78 external components as we are in the process of fully releasing the TPS7A78 device and is full detailed datasheet in the next few months.

Hope this helps for the moment and please could you share your customer use case and schematic (if you perfer to this offline) since HB configuration does not require a diode to be connected in series with CS and RS components as they are connected to either AC+ or AC- pins.

BR,

Haidar