ST25R95: can VPS_TX & VPS be different; eg, VPS_TX = 5 V, VPS = 3.3 V ?

[stst9187]

Hi,

Can the VPS_TX pin be powered with a voltage different from VPS? Specifically, can you connect VPS_TX to 5 V while VPS is at 3.3 V?

I think it should work, but I would like your confirmation as I couldn't find this information in the datasheet.

Thank you for your support.
Marco

[Brian TIDAL]

Hi Marco,

Could you provide more details about the reason to connect the VPS_TX pin to 5 V? 

See also the following post related to VPS_TX at 5V: https://community.st.com/t5/st25-nfc-rfid-tags-and-readers/using-vps-tx-5v-on-st25r95/td-p/145420.

If you need to increase the reading distance, use the ST25R200 device, which provides higher performance.

Note that the ST25R95 is Not Recommended for New Design (NRND).

Rgds

BT

[stst9187]

Hi Brian,

Thank you for your quick reply.

Basically, my customer developed a product that is powered at 5 V (both VPS_TX and VPS). This product has been manufactured for some years, and now they need to develop a new hardware version.

I suggested to the customer to reduce VPS_TX (from 3.3V to 5V) because the power dissipation is too high. I built a prototype and measured a decrease of about 10 % in distance with VPS_TX = 3.3 V.

My intention is to provide both options in the new hardware for backward compatibility. I plan to add two 0 Ω resistors to select whether VPS_TX is connected to 3.3 V or 5 V.

This is the scenario. I look forward to your feedback.

Best regards,
Marco

[Brian TIDAL · Accepted solution]

Hi Marco,

VPS (also called VPS_main) and VPS_TX are independent power domains. VPS can be 3.3 V, while VPS_TX can be 5 V.

A voltage regulator is likely to be added to the board to provide a 3.3 V power supply. The reduction in power dissipation in the ST25R95 is likely to be compensated by the power dissipation in the regulator.

I draw your attention to the change in differential impedance value between 5 V and 3.3 V: see Zout in Table 52  Reader characteristics in the Data Sheet. This may impact the existing matching circuit. Make sure to verify the matching circuit at 3.3V.

Rgds

BT