NFC for wearables to enable reliable contactless payments

October 22, 2015 // By Graham Prophet
ams’ AS3921 is an NFC transceiver with the company’s boostedNFC technology: it has ultra-small footprint and very low power consumption, required for the latest generation of wearable and portable devices.

Aimed at manufacturers of smart watches, wristbands and other space-constrained devices, and enabling them to reliably support contactless payments and ticketing, AS3921 is an NFC analogue front end (AFE) with ams’ boostedNFC technology, which increases the useable operating volume of an NFC reader by up to 900% when compared to conventional NFC implementations. This greatly improves the reliability and perceived speed of NFC transactions, particularly in devices such as smart watches and wristbands that only have room for an extremely small antenna. It also ensures reliable NFC transmission in other device types which have a smaller antenna than a contactless card’s, or which present difficult operating conditions for NFC transmissions.

Optimised for space-constrained assemblies, the AS3921 enables OEMs to implement a simpler design than the conventional NFC controller-based architecture allows, resulting in a simpler and cheaper solution, occupying a board footprint around one third smaller, using fewer components and consuming less power.

Drawing 12 µA in normal operation, and providing a power-saving Secure Element wake-up function, the AS3921 drains less power from the battery than NFC controller circuits, which typically draw 60 µA or more. Packaged in a wafer-level chip-scale package (WL-CSP) measuring 2.115 x 1.735 mm, and connecting directly to the Secure Element with few external components required, the IC implements active load modulation: it generates an RFID card response which is synchronous to the reader’s field. This allows for card-to-reader communication at coupling factors as much as one order of magnitude lower than is possible with the passive load modulation (PLM) used by contactless cards.

A device which uses the AS3921 and which has an antenna of just 100 mm ² can achieve the same or a bigger operating volume as a contactless card with a conventional PLM circuit, transmitting via an antenna typically at least 2,150 mm ² in area. In some cases, the device might need no dedicated NFC antenna at all, provided its metal casing