ST25DVxxx V_EH output voltage

[Marco Burgener]

Posted on July 02, 2018 at 11:04

Hi

I'm playing around with an ST25DV_Discovery_ANT_C6 and an ST25R3911B-DISCO development board to test the energy harvesting features. One result of my tests yields, that the output voltage V_EH is limitted to approx. 3.4V eventhough I can't find any explenation therefore in any application note or datasheet.

R = 10kOhm

In the datasheet of the ST25DVxxx is written, that the voltage V_EH is NOT regulated. If that's true, I'm not able to understand my measurements.

Do you have any explenation for this behavior?

Thanks and best regards

Marco

energy-harvesting st25dv64k st25r3911b-disco

[JL. Lebon · Accepted solution]

Posted on July 09, 2018 at 14:57

Dear Marco,

First, thank you for considering the ST25DVxxx.

Your measurement is actually correct.

The V_EH output is indeed not regulated, but is internally limited.

The V_EH output directly depends on the the voltage applied on the AC0/AC1 antenna coils.

Voltage on the antenna coil can rise up to high values (much more than 10V) if RF field power is strong.

In order to protect the tag from those high voltage on AC0/AC1, there is a internal clamp mechanism which limits this voltage to values that the tag can support. As V_EH output voltage directly depends on AC0/AC1 voltage, it is also limited by this clamping value.

This is the reason why you see a limitation around 3.4V (the clamp is actually not perfect and with very high RF filed power values, it is possible for the V_EH voltage to rise above 4V).

You can see in the application note AN4913 in figures 3 and 4 that the V_EH voltage is limited to values between 3V and 4V. This is due to the clamp. When the RF field power is above 6A/m, the clamp let the voltage rise a little bit and you can have V_EH>4V.

Hope this answers your question.

Best regards.