Layout Recommendations for ST3916 and ST3918

[ELTE-Suraj]

We are currently seeking layout recommendations for the ST25R3916 and ST25R3918 chips.

On the Transmitter Side:

ST25R3916B → Power Amplifier → Matching Circuit → Antenna

On the Reciever Side:

Antenna → Matching Circuit →Power Harvester → ST25R3918

1. The implemented power amplifier is a Class E amplifier configured in push-pull mode  (We intend to transmit power along with NFC communication)
2. The matching circuit has been designed using the ST25R Antenna Matching Tool.
3. We are using a differential configuration, and as such, the matching elements have been placed symmetrically on both arms.
4. A provision for a balun has been included between the power transmitter and the matching network.

We have the following queries:

1. The ST25R Antenna Matching Tool provides both single-ended and differential matching options. For the single-ended configuration, we understand that the PCB trace impedance should be matched to 50 ohms, whereas for the differential configuration, the PCB traces should follow 100-ohm differential routing.Could you please confirm if this understanding is correct?
2. If we choose the differential configuration, should the entire signal path—from the ST25R3916 to the antenna—be routed with 100-ohm differential impedance?
3. Are there any application notes from ST regarding power amplifier design? At present, we have designed the amplifier based on standard Class E equations and have simulated the circuit in LTspice.  However, if there are any specific resources or recommendations from ST, we would be grateful if you could share them. 

[Travis Palmer]

Hello  ELTE-Suraj,

May I ask, which application you are targeting? 

We provide a general Application note for PCB layout of our ST25R devices.

For the ST25R3916(B) and ST25R3918 it is AN5240. 

Currently we do not have a reference design for an external amplifier. The issue is, that a normal class E amplifier can only do OOK modulation (ISO14443-A and ISO15693) but is not capable of doing AM Modulation (ISO14443-B and Felica). 

It is not required to design the PCB traces for the matching network at a certain characteristic impedance. The square signal coming from the driver is only present between RFO pins and EMI filter. between EMI filter and series cap and further towards the antenna, mainly the 13.56MHz is present.

The Lambda/4 is way below the typical trace length between RFO pins and EMI filter. Our recommendation is that the EMI filter is placed as close as possible to the IC. Even when considering the 100th harmonic of 13.56MHz the trace length should be way below.

Please let me know, if you have further questions.

br Travis

[ELTE-Suraj]

Hello,

Thank you for your response.

The application involves powering a slave device using a coil (Wurth 760308101150). Therefore, the antenna (NFC coil) will be connected to the PCB using copper wires approximately 20 cm in length.

From your reply, I understand that there is no need to match the characteristic impedance (50 ohms) on the PCB itself.

However, I believe the impedance seen from RFO1 and RFO2 must be matched to 50 ohms., 

For instance, when I tune the network using a VNA, the impedance at 13.56 MHz should be as close to 50 ohms as possible. Is that correct?

If there is no such requirement for 50-ohm impedance matching in NFC applications, please let us know.

[ELTE-Suraj]

Just a gentle reminder — I wanted to kindly follow up on my recent query, as I haven’t received a response yet. I would appreciate it if you could take a look when you have a moment

[Travis Palmer]

Hello  ELTE-Suraj,

to match to a certain target matching impedance (e.g. 50 Ohm) does only make sens if your driver has similar impedance. Only exception is, when using designs with an single ended antenna with 50Ohm cable.

The driver impedance of our ST25R chips is typically < 2Ohm. It would mean, that your matching network should be also around 2Ohm. In this case 50% of the power will be dissipated inside the ST25R and 50% will be delivered to the matching network. But efficiency will be around 50% and the resulting total power consumption (I_VDD ~ 772mA) will be out of spec. 

We recommend (AN5276) to use the lowest driver resistance by having a higher target matching impedance (e.g. 15Ohm). This way, the efficiency will be ~70% while still having sufficient output power. 

The applications where someone needs to go to the maximum of I_VDD (e.g. 350mA / 500mA) are very rare but the chip can handle it.

Please feel free to choose the target matching impedance dependent on your application. No matter if 2Ohm or 50Ohm the IC can handle it as long it is operated withing the absolute maximum ratings, and operating conditions specified in the datasheet. 

In my opinion taking care about reflections and characteristic impedance does not make sense for 13.56MHz and trace length < 1cm. The only exception is the connection between RFO pins and EMI filter (=> rectangular broadband signal). 

I think it might be interesting for the community to follow your experiments and conclusion. Please share the results within the community and please let me know if you have further questions. 

br Travis

[ELTE-Suraj]

Thank you for the detailed explanation.

We will begin sharing our observations once we start the actual testing on the mounted PCB. We plan to proceed with the PCB order shortly.

As part of our design, we are creating a separate PCB dedicated to the power amplifier, matching circuit, and EMC filter. This board will interface with the ST25R3916 through the RFO1 and RFO2 outputs, and receive inputs via RFI1 and RFI2.

Could you please advise on the recommended number of PCB layers for the antenna layout?

Regards

Suraj

[Travis Palmer · Accepted solution]

Hello  ELTE-Suraj,

For the ST25R3916 section a 4 or more PCB layer are recommended.

For the pure antenna typically 1 or 2 layer are sufficient, dependent on the complexity of the antenna and if you are able to route it. 

As an example the ST25R3916-DISCO can be taken.

In addition a Layout recommendation application note is available (AN5240). 

BR Travis