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 & PTM_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, correct?  My thoughts are that the matching network for maximal power transfer should also be matched to 1.7 ohm. Is this accurate?
    • 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. 

[Travis Palmer · Accepted solution]

Hello Wrig0421,

It is not clear for me which role the ST25R3916B will play?

Do you want to use it as a transmitter or as a receiver?

If Tx only, yes you can use the AN5276 as a guideline for matching.

What will be used for the Rx device? I propose that the whole matching is done when both (Tx and Rx) antennas are at their final place.

Regarding your questions:

Transmitting with or without modulation will not significantly impact the power transfer.

add 1.) What you are describing is not possible with the device. The Driver current through the transmitter should not exceed 350mA, as stated in the datasheet. It is possible to bypass the internal regulator which allows the driver current up to 500mA. Matching the antenna to the same impedance as the driver will cause a current above 600mA at an efficiency of ~50%. Which means, if you output 1,7W also 1,7W will be dissipated by the device. At a thermal resistance of your PCB of ~60°C/W it means a self-heating of ~102°C.

add 1.2.) There is no internal amplifier. The ST25R typically consists of an internal LDO which generates the driver voltage (VDD_DR). The normalized driver resistance is typically set to 0 which is the minimum and gives the highest efficiency.

The calculation of the dissipated power in the device can be found in AN5584.

add 2.) The impedance mentioned in the AN is an example. Some customers want to spend more power, some less. The AN should guide them how to select the matching impedance.

Add 4.) pt_res and ptm_res is the driver resistance in card emulation mode. (=> are you using the device as Rx device?) For Reader mode d_res must be considered. pt_res and ptm_res do not have an impact.

Add 5.) No, See question 4.

Your questions:

• Yes, lowest d_res should be chosen and is already chosen per default.
• Maybe not feasible due to the current limits mentioned in DS13541 Rev 10, Table 122.

I would recommend starting at a Matching impedance of 9Ohm. The current consumption should be right below 350mA and the internal regulator can be used.

You may also want to look at a more recent device. The ST25R300 can output 500mA continuously at 6V VDD which allows an output power of around 2.2W and a more efficient driver resistance.

BR Travis