ST25DV: why is a DC voltage delivered on VCC?

[JDirk.1]

An internal voltage regulator allows the external voltage applied on VCC to supply the ST25DVxxKC, while preventing the internal power supply (rectified RF waveforms) to output a DC voltage on the VCC pin.

Whats the purpose to output a DC voltage on VCC?

I measure 2,3 Volts on VCC once the power is off. Whats the purpose of it? I guess its delivered by the I2C bus?

[JL. Lebon · Accepted solution]

Hello,

The ST25DV can be powered either by the VCC pin or by the RF interface (rectified waveforms).

When powered from RF, the internal regulator is not used. The rectified waveform provides voltage for the RF interface and internal logic. The I2C interface is not available.

When powered from VCC (a DC voltage is applied on VCC pin), the regulator provides the voltage for internal logic and the I2C interface. The RF interface is not available.

If both RF field is present and VCC is connected to a DC source, then the power coming from RF is internally disconnected and only power coming from VCC pin and internal regulator is used internally.

Nevertheless, due to some leakage in some regulator's transistors, it is possible that power coming from RF leaks through the VCC pin if this one is left unconnected. This may be the reason why you can measure some voltage on VCC pin. This has no impact on the device behavior.

I recommend not to let the VCC pin floating is you do not intend to use it.

Best regards.