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Layout Recommendations for ST3916 and ST3918

ELTE-Suraj
Associate II

‎2025-04-09 5:50 PM

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. 

 

 

 

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2 REPLIES 2
Travis Palmer
ST Employee

‎2025-04-10 12:43 AM

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

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ELTE-Suraj
Associate II

‎2025-04-10 1:18 AM

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.

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