When to use external load modulation on ST25R3916B?

[smithderek2000]

Hello,

The datasheet mentions how to use external load modification on the ST25R3916B but it doesn't clearly describe *when* to use it. Under which circumstances does it make sense to add the diodes and FET for external load modulation?

Thanks,

Derek

[Travis Palmer]

Hello Derek,

The ST25R3916(B) offers internal measures (Register 29h) to achieve sufficient LMA fitting for various standards (ISO, NFC Forum and EMVCo). In Register 29h pt_res and ptm_res defines the driver resistance for unmodulated state (ST25R3916 is not transmitting in CE mode) and modulated state (ST25R3916 is transmitting in CE mode).

The delta between pt_res and ptm_res is defining the strength of the LMA. E.g. Register 29h set to 0xF0 means a strong LMA whereas Register 29h set to 0xFE will produce a weak LMA.

The setting of pt_res also has significant impact on the loading effect. For most matchings and antennas a setting of 0xF delivers best results.

A good strategy is to execute the loading effect tests with different settings of pt_res (e.g. pt_res set to 0, 7, B, F and compare the results).

The generated LMA is also dependent on used matching components/values, antenna size and also exposed field strength.

If the Register 29h cannot deliver strong enough LMAs, the ext_LM circuit can be used in addition.

The LMA strength can be adjusted by changing the resistance value (default 100R).

The LMA can be used instead or in addition to the internal restive load modulation.

Also the polarity of the EXT_LM can be changed, which, when be used in combination with the internal restive LM can be dangerous. Both mechanisms could cancel itself when the wrong polarity is being selected.

I hope, this explanation helped you a bit. For further information it would be helpful to know more details about your application. What is your antenna size. Which standards do you have to comply with? What are the results you are achieving and what are the goals you would like achieve?

BR Travis

[smithderek2000]

Hello, thanks for the response. The application for this is a pharmacy bottle reader. It is a 16 channel RFID tag reader; with each ST25R3916B going through eight DPDT relays to the antennas. Antennas are PCB trace coil, about 0.75 inch in diameter, spaced out about 1.5 inches on center from each other. The board is big; 24 inches long by 3 inches wide. Read distance is very close, 1-2cm at most. Regarding standards, just FCC now on the emissions. On the tag side we are looking at both 14443 and 15693 tags for the application. I didn't put the external load modulation components on this design because I didn't see them on any of the reference designs. I'm mainly using x-nucleo-nfc08a1 as my example.

The application is a little bit different from normal readers, we will be using the relays to be switching between each coil to read the 16 different positions. We don't need to switch very quickly; maybe 1 second between each. I figure that will give us enough time to retune the antenna match circuit for each channel.

Any other advice?

[Ulysses HERNIOSUS · Accepted solution]

Hello Derek,

if your application is only acting as a reader/writer, then you will not need anything on EXT_LM. These only play a role when using the Card emulation feature of the ST25R3916B. Actually X-NUCLEO-NFC08A1 has these components mounted: Q300, etc. on bottom side.

Best Regards, Ulysses

[ClaRos91]

Hi Ulysses,

is there a simple way to enable the EXT_LM signal in the STEVAL-ST25R3916B board through the ST25R3916 Discovery GUI? In the Card Emulation tab I didn't find any option. We have to modify the firmware and reload it?

Thank you

Cla

[Ulysses HERNIOSUS]

Hi Cla,

this can be easily done via the GUI, just read from board or load the default file and adapt as per this picture. The setting for Ext_LM is inside CHIP_INIT:

When writing to the board you have also the option to put it into MCU flash to be used as default.

Regards, Ulysses