Last year, about 40% of new smartphones included Near Field Communication (NFC). Analysts predict that by 2017 there will be 1 billion NFC enabled phones. Clearly, the use of NFC is ramping up because it can simplify aspects as diverse as communication, secure payments, user authentication, and retail loyalty programs for instance.
Adding NFC functionality to an integrated circuit involves connecting the wireless interface pins to an antenna/coil. The voltage on those pads strongly depend on the distance between and alignment of transmit/read devices and the power of the transmitting device. Simulations by one of our customers showed a worst case of almost 10V between pads, which is rather high for advanced CMOS technology.
There are basically 2 ways to cope with this excess voltage.
Use high voltage transistors
Designers can use transistors that tolerate voltage up to 10V. Fortunately, many NFC applications also require on-chip non-volatile memory (NVM). Such NVM circuits typically use a thicker gate oxide and foundries provide high voltage transistors for such embedded flash process flavors. Several of Sofics’ customers have used those NVM transistors for the first stage of the antenna circuit. The problem is that high voltage transistors are easily damaged during electrostatic discharge (ESD) stress. A parallel on-chip ESD protection circuit (with 10V tolerance) is the easiest solution. Sofics has ESD clamps on several technology nodes that are used for the protection of NFC antenna pads.
Limit the voltage
It is possible to reduce the excess voltage with a so-called clipping or limiting circuit. The simple approach is to use a set of diodes but due to the large amount of current the diode perimeter is rather large. This leads to high leakage current during non-clipped operation. Sofics has designed a novel clipping circuit to solve those issues. The clipping circuit was used to protect the NFC antenna pads in an ultra-low power Bluetooth chip with ‘Touch-to-pair’ functionality. The product, processed in TSMC 55nm technology, is running in mass production and is used for all kinds of home automation and IoT applications.
Sofics’ clipping circuit is designed in a TSMC 55nm technology for a Bluetooth device with on-chip NFC for “Touch-to-Pair”. The total area is 5500um² for a maximum current of 100mA. The clip-level can be set at 3.6V or at 2.2V. The clipping circuit can also be disabled when the pads are used as a general 1.8V General Purpose interface (GPIO).
Without excess voltage limiting circuits the voltage can get close to 10V. The Sofics’ clipping circuit limits the voltage between the +/- antenna pads to 3.6V during functional operation. The clip-level can be set at 2.2V for compatibility with low power 1.8V interfaces too.
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