M24LR04 energy harvesting shuts down with certain phones

[CLars.1]

We are evaluating the M24LR04 for a specific application where energy harvesting is required. We have configured the energy harvesting mode 3 to ensure that it will attempt to continue to harvest with low magnetic field strength. However, we regularly see the power supply shut down with certain phones. The Nexus 5, for example, will only harvest energy in a very narrow range between 5 and 8 mm distance and in a very narrow X, Y alignment window.

Is there anything we can do to help this? Why does the chip shut down energy harvesting at all? I would sooner let Vout sag than shut it down completely. Is there a different chip that we can use that will have better performance?

[Ulysses HERNIOSUS]

Hi,

there has been some discussion on a similar topic on a similar chip here:

https://community.st.com/s/question/0D53W000004Gq7cSAC/low-energy-harvesting-voltage-on-a-custom-st25dv-tag

Maybe it helps.

REgards, Ulysses

[CLars.1]

Thanks for the very quick reply. We have extensive experience in designing and tuning NFC antennas, and are harvesting energy over a wide range of X, Y and Z with this same phone/antenna/tuning in the same conditions using ICs from different manufacturers. However, the M24LR04 will only operate in EH3 with the Nexus 5, and even then it only harvests in an insanely tight window. I can read the M24LR04 from 2 cm+ distance, but I can't harvest energy until I'm between 5 and 8 mm distance and within probably +/- 2 mm in X and Y. Maybe that's just the nature of the M24LR04. Are any other IC's expected to have different (better) harvesting behavior? It's almost as though we need an EH4 that doesn't ever turn the harvesting off, regardless of the RF field strength. Since the rest of our circuit can operate down to 1.7 V, we would prefer the IC continue to operate at lower voltages rather than turning off harvesting when it can no longer achieve 3.3 V.

We can try the on-demand harvesting mode, but I am concerned that if it is disabling harvesting since the RF field isn't strong enough that dynamic harvesting control won't help significantly. I am monitoring the voltage from Vout and I find that it only reaches 3.3 V in the very narrow window described. I don't believe that turning the harvesting on and off will change that - will it?

[JL. Lebon · Accepted solution]

​Hello,

It all depends on how much power you sink on the EH output.

With EH set to 3, you can sink ~10mW if the RF field is above 3.5A/m and only 0,69mW if H is at 1A/m.

If you exceed the max output power allowed, for example sink 5mW but have only 1.5A/m, the EH will be cut since there is not enough energy on antenna to power both M24LR and the device connected on EH output.

The M@$LR has this special mechanism that protect communication capabilities over EH, by delivering EH only if there enough power to first power the internal logic of the tag.

If you are looking for a tag that do not have this kind of behavior, I can suggest to try the ST25DV-I2C. This tag deliver EH as soon as there is enough power to power the tag, and don't have the caliber settings as in the M24LR. The experience may be smoother

Best regards.

[CLars.1]

Thanks - the ST25DV-I2C looks like it is probably what we're looking for. We'll test it out.