ST25DV04K: Low current on V_EH pin vs Pulse Load requirements (7mA). How to size the bulk capacitor?

[Matheus_Markies]

I am working with the X-NUCLEO-NFC04A1 expansion board (based on the ST25DV04K) and I have some questions regarding the Energy Harvesting (EH) capabilities.

My Setup:

Hardware: X-NUCLEO-NFC04A1 connected to a host MCU.

Observation: When measuring the V_EH pin directly, I see a very low continuous current (in the 0.3µA range), which seems insufficient to drive my target peripheral directly.

My goal: I am trying to trigger an external circuit that requires approximately 7 mA for a period of a few ms. I understand that the ST25DV cannot supply this current continuously, but I have seen applications that use a reservoir capacitor to obtain high pulse charges that can be used to trigger the system.

My questions:

Is the low DC current at V_EH normal? Was the V_EH pin designed strictly to slowly charge a capacitor, rather than directly drive a load?

Is it possible to increase the EH current? In addition to optimizing the antenna adjustment and coupling distance, is there any setting in the ST25DV that affects the drive strength of the analog collection block? Or on the reader side, I am using the X-NUCLEO-NFC12A1.

Solution sizing: What is the recommended method for calculating the charge time in relation to the capacitor size? Given my peak current and the time it will be active.
Example: If I need to maintain 7mA for 10 ms (dropping from approximately 3.2V to 2.2V), how can I estimate how long it will take the ST25DV to recharge this capacitor between pulses, given the limited current of the RF field?

Any application notes or formulas related to output impedance V_EH would be very welcome.

Future ideas based on the theoretical results above: My idea was to use a CPH3225A supercapacitor, which has 11mF. The idea is to validate this component for this system with consumption and charging calculations. To manage charging, I would use a BQ25570RGRR power management IC.

Thanks in advance!

[JL. Lebon · Accepted solution]

Hello, 

This is a late answer, but it may still be useful to you.

The 0.3uA current you are reading is not what is expected out of the V_EH pin when EH is enabled, so maybe there is something wrong in your configuration. Actually, the ST25DV04KC is expected to provide around 20mW in best conditions.
To enable the EH, you have two ways: static register EH_MODE or dynamic regsiter EH_CTRL_Dyn.
Using the static register, you enable the EH at every boot: as soon as there is enough RF field power to power the device, you should have power out of the V_EH pin. The drawback is that if you sink too much current on this pin immediately, and if the field is just enough to power the tag, you will go to a reboot loop. This may be what you experience.
Using the dynamic register EH_CTRL_Dyn, you can enable EH on demand ahen you know the RF field is already enough. The drawback is that if you sink too much current and cause a reboot, you need to re-enable EH wirting the EH_CTRL_Dyn again, as this is a volatile regsiter.

Concerning the tank capacitor you want to implement, this is possible and can allow you to absorb peak current. What you need to do is to limit the charge speed of the capacitor so that it doesn't sink too much current when charging (adding a resistor you can calculate the RC timing). This will cause the charging time to increase but will protect from rebooting the tag by limiting the current sink on the V_EH.

Best regards.