Touch switch chip includes power management

EDN Europe, 20 Aug 2007

20th August 2007 - Quantum Research has developed its capacitive touch sensing switch technology in a number of different forms. Based on what it terms a charge-transfer technique it has the QTouch offering for simple single-point touch detection; QMatrix to provide a key panel with multiple touch zones; and QWheel/QSlider for, as the names suggest, rotary and linear variable-control inputs. Now, the company has augmented the first of these, the QTouch range, with the QT102 capacitive touch switch stand-alone IC. Previously, Quantum has implemented its switching technology as a combination of mixed-signal front-end technology combined with microcontroller code that carries out the detection and a range of other functions. Users would license the code as part of the technology purchase, to build into the rest of the code running on the host MCU of their product. As well as detection of an activating touch, the code continuously calibrates the capacitive sensor, compensating for short- and long-term environmental changes and drift. It also adds features such as on/off toggling to the touch detection function. Quantum designed the QT102 for simple applications where the product designer may not wish to ad code to the host controller; or where the code space is fully utilised; or where there may not be a host. It performs single-point touch detection, with the toggle function for touch on/touch off operation, and includes power management and safety features. For use in appliances, the chip implements a safety-timeout, programmable by setting a resistor value, that turns off the appliance after a set period (typically 15 minutes). You can opt to have the time-out interval automatically restarted if the appliance is used during that interval. The chip operates autonomously, without the need for an external microcontroller (Quantum says that it does in fact embody a small microcontroller core, running code derived from the company’s other products), and directly drives the gate of a power-switching MOSFET: you can set the output to drive high or low. As with other Quantum switches, the code suppresses false triggers from noise or transient brushed by passing objects. The basic circuit configuration adds – in addition to the touch sensor – three resistors and one capacitor. The QT102 comes in a 6-pin SOT-23 package: it runs at a stand-by power level until a user touches its sensor, consuming a typical 37 uA at 3V. It costs under $0.50 in volume, and a demonstration board costs $10. As a firmware-based device, Quantum can produce customised versions for high-volume applications.