Power supply to ST25DV-I2C for GPO output and I2C communication.

[KT_25]

I would like to use ST25DV-I2C for I2C communication and GPO output.

The datasheet states that to use GPO output, an external pull-up is required for the 8-pin package, and an external power supply (VDCG) is required for the 12-pin package.

However, I do not want to have an external power supply on the board.

Is it possible to transmit power to the ST25DV-I2C via RF, and then supply power to the pull-up or VDCG from V_EH?

Also, can power be supplied from V_EH to VCC for I2C communication?

[JL. Lebon · Accepted solution]

Hello, 

In order for the I2C interface to work, there must be a power present on the VCC pin. 
If you don't have an external VCC source, you can connect the V_EH output to the VCC input pin and enable the energy harvesting. In this case, you will be able to power the device from its VCC pin only when an RF field is present and the I2C interface will be functional.
Keep in mind that I2C is an open drain bus and requires also pull-up resistors in order to work. Those pull-up resistors can be connected to a different power source than the ST25DV VCC, as long as this power source is in the range 1.8/5.5V.

For the GPO output, if you are using 8-pin package, it requires an external pull-up resistor as this output is open drain. You can also connect this pull-up resistor to the V_EH or to any other external power supply source that is in the 1.8/5.5V range (it can be different than the St25DV VCC voltage).
It is the same thing if you are using the 12-pin package: you can connect VDCG to V_EH or any other external power source.

Best regards.

[KT_25]

Thank you for your answer.
I have an additional question. I'm having trouble choosing between the 8-pin package and the 12-pin package. What is the difference between the two packages?
Also, are there any differences in the uses for each package?

[JL. Lebon]

Hello, 

There are 2 differences:

First is the nature of the GPO pin output.
- The GPO output of the 8-pin is of Open Drain type. This means that it is active low and requires an external pull-up resistor.
- The GPO output of the 12-pin is of Push-Pull type. It is active high, doesn't require an external pull-up, but requires an additional power supply (the VDGC pin, that is powering only the output stage of the GPO pin).

Second difference is that the 12-pin package has an additional pin called "LPD". This pin, when set to high level, can put the device in a low power mode where current drawn on the VCC pin is less than 1uA. The 8-pin package can't do that.

In your use case where you want to power the Vcc through V_EH, I think you have no use of the LPD pin (it is useful only in an application where VCC is permanent). So, I think you can go with the 8-pin package.

Best regards.

[KT_25]

@JL. Lebon 

>> In your use case where you want to power the Vcc through V_EH, I think you have no use of

>> the LPD pin (it is useful only in an application where VCC is permanent).

>> So, I think you can go with the 8-pin package.

Thank you for your detailed explanation. Please tell me more about this part. If power is supplied from V_EH to VCC, why not use the LPD function?

I have two additional questions about the following:

(A) If the LPD function is used, does it affect GPO output, I2C communication, or NFC communication?

(B) According to the datasheet, when the LPF pin is set to High, the internal 1.8V regulator is turned off. What is this regulator used for?

[JL. Lebon]

Hello, 

The intent of the LPD pin is to reduce power consumption on the VCC when the device is not intended to communicate through I2C.

When LPD is high, it is not possible to communicate though I2C anymore. This is the only restriction. The GPO output is not affected and still works if there is an RF field present and if there is a VDGC power supply. The RF communication is still possible.

If the RF field and VCC are both present and LPD is high, the device is powered through the RF field.
If the RF field and VCC are both present and LPD is low, the device is powered through the VCC (though an internal voltage regulator).
If the RF field is absent and VCC is present and LPD is high, the device is NOT powered.
If the RF field is absent and VCC is present and LPD is low, the device is powered through the VCC.

In your application where you want to connect the V_EH to VCC, I guess you don't need LPD since when RF field is absent, there is nothing on VCC, so no need to go to low power mode. And if you don't want to power the VCC when RF field is present, you just have to disable the energy harvesting.

Best regards.

[KT_25]

I now understand the LPD function well.
Thank you for your detailed explanation.

I have one additional question.
If there is no supply to the VCC of the tag IC, can RF communication be performed with the reader IC?
(For example, communication in which the reader IC authenticates the tag IC.)

[JL. Lebon]

Hello, 

"If there is no supply to the VCC of the tag IC, can RF communication be performed with the reader IC?"

Yes absolutely. In that case, the ST25DV is entirely power by the RF field and all RF communication are working. Only the I2C bus is not functional. You can read the UID, read and write into EEPROM etc. from the reader.

Best regards.

[KT_25]

I understand that RF communication is possible.
Your detailed answers to my multiple questions were very helpful.
Thank you very much.