ST25R3916 - how to replace the variable capacitors if we do not need card emulation mode

[smithderek2000]

Hello, we have a working design that is in production, but obviously cannot keep building because of the EOL variable capacitors. Our application is reader only, no card emulation. Since we don't need CE, do the variable capacitors need to be populated? Or can they be replaced with a fixed value capacitor? If the latter is correct, then what value capacitor to use? We want to minimize changes to the design since it is working well.

In the future we will change to the ST25R300 bur right now we have to stick with the current design.

Thanks,

Derek

[Ulysses HERNIOSUS]

Hi,

typically you will have static MLCs in your matching which need to be adapted to compensate the un-populated variable capacitors. Only a BOM change will be required.

BR, Ulysses

[Travis Palmer]

Hello Smithderek,

As Ulysses stated, the VCCs (voltage controlled capacitor) are mainly required for switching between reader mode matching and card emulation mode matching. An alternative would be described in AN5896. 

Since you do not require card emulation, you can just remove them. The MLCC value depends on the used setting and used VCC type. At the end you have to select the MLCC value to end up with the same matching as before or a matching for normal reader configuration as described in AN5276.

For the same reason as R3916(B) the R300 offers antenna tuning. It is based on external MOS transistor and MLCC capacitor. It is not required for RW only operation. It is one option for changing between reader and card emulation matching or to mitigating component and production tolerances.

BR Travis.