Wireless interfaces face excess voltage
A few years ago, no more than a handful of smartphones supported Near Field Communication (NFC). Now, 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 depends on the distance between and alignment of transmit/read devices and the power of the transmitting device. Measures need to be taken to protect the wireless interface from excess voltages.
- Let Sofics protect your wireless interfaces
Limit the voltage between antenna pads during functional operation
There are basically two ways to cope with the excess voltage:
- Use high voltage transistors
Designers can use transistors that tolerate voltages up to e.g. 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 applications. Several Sofics
customers have used those NVM transistors for the first stage
of the antenna circuit. However, 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 in several technology nodes that are used for the protection of NFC antenna pads.
- Limit the voltage
However, the high voltage tolerant transistors are not required: 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 would be rather large. This leads to high leakage currents 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
in mass production and is used for all kinds of home automation and IoT applications.
- Sofics clipping circuit protects wireless interface circuits against excess voltage
Programmable clipping circuit
The 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 2.2V. The clipping circuit can also be disabled when the pads are used as a general 1.8V
General Purpose IO (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.
- Sofics clipping circuit can be ported to any technology and any specification as is required