ST25R3916B Maximum Unmodulated Power Transfer Antenna Design + Match

[wrig0421]

Hello,

I am back to working on testing a custom PCB with the ST25R3916B chip included on it!  This custom hardware is setup for the differential antenna connection.  The coil to be attached to this custom PCB is different than the one included on the eval board for the ST25R3916B chip.  The pads for the matching components are all included but the components still need to be calculated for the coil.  I have access to a VNA and can follow the procedure for matching... 

Regarding the matching component calculations...I have reviewed the AN5276 application note and have some questions &/or statements to confirm.  In general I don't fully understand the "target impedance" to match antenna to.  For my application I'm trying to maximize power transfer through a wire wound TX coil.  

1. Under section 3.2 in AN5276..."The impedance matching is the most important factor to determine the power consumption...higher target matching impedance less power is transferred forward to the antenna, and therefore the power consumption of the whole reader unit is reduced."
   • For my particular application... The wireless energy is ONLY being used to power an external board.  There is no modulation [no communication] in my application.  Simply constant unmodulated power transfer. 
     • Given the above statement... My thought to maximize the energy transfer is by doing the following...Can you confirm my thoughts below are correct to maximize energy transfer?
       1. Matching the antenna to the output impedance of RFO1 RFO2 driver output resistance.  
       2. Lower the output impedance of the output stage to increase current through coil.  
          • Technically register 0x29 [section 4.5.50] can be used to set the normalized output resistance [... actual value = setting * output resistance in section 5.4].
          • I'm wondering if there's a concern of over driving the internal power amp with specific setting combinations?  
2. Still under section 3.2 in AN5276... "The smith chart in Figure 4 shows the recommended matching impedance as a starting point in reader mode".  The smith chart shows an impedance of approximately 14.373 - j0.20585 ohm.  
   • Is this simply an example or is this truly the recommended matching impedance?  
3. Under section 5.2 in AN5276 it's stated that "the value to be targeted in loop antenna design for NFC reader applications is 0.2 to 1.5 uH range."
   • Understood.  My coil shall fall in this range for testing.  
4. Under section 6.1 in AN5276 "In the precondition tab, enter the desired EMC filter cutoff frequency, the target matching impedance, and the desired antenna Q factor."  
   • The target matching Z field shown in Figure 19 shows "Target matching Z" is set to 10 ohm.  That's fine.  Just potentially not setup for maximum power transfer.  However, this is dependent on the PT_RES setting in reg 0x29.  
5. Under section 6.2.2 in AN5276 "the output resistance is set to 2 ohm differential and the matching network is tuned to (10.96 - j0.9248) ohm." 
   • My understanding is that this is potentially not setup for maximum power transfer.  However, this is dependent on the PT_RES setting in reg 0x29.  

My main question is the following: 

• For my application where I'm trying to maximize power transfer only [no modulation]...
  1. My thought is to set the output resistance to as low as possible. 
  2. Match the antenna network to the output resistance value set in the above... The calculated one from setting * typical resistance. 
• From section 5.4 in datasheet... 
  • RFO1, RFO2 has a typical value of 1.7 ohm and max of 4 ohm. 
    • If PTM_RES & PT_RES set to 0 then the driver output resistance typical is 1.7 ohm.  Therefore the matching network for maximal power transfer should also be matched to 1.7 ohm. 
    • Given the damping resistor included in the design..  I'm wondering if there's a higher output resistance value that is more optimal for maximum power transfer. 

For maximum power transfer... My main questions are the following:

• What output resistance should be set for RFO?  1.7 ohm typical * <insert normalized PTM_RES & PT_RES factor here>. 
• Given the damping resistor and filter... is targeting impedance of say 8-12 ohm better for maximum power transfer? 

Datasheet download link

AN5276

Thank you in advance.  Looking forward to response.