EDNE April 2012 - (Page 49)

Noise wars: Jo h n Car e y • C y p r e s s s e m i C o n d u C to r C o r p PRojected cAPAcItAnce stRIkes bAck AgAInst InteRnAL noIse oday’s users expect multitouch systems to perform with precision and still comply with demanding environmental standards. Designers face no small feat in meeting these requirements. With a rapidly changing internal environment in multitouch systems, the war for touchscreen dominance is effecting the emergence of new battlegrounds. One current trend is the push toward thinner phones. Achieving this goal means direct lamination of capacitive-touch sensors to the display, moving the sensor inside the display, and overcoming many other challenges with T ≃220V (a) antennas and ground loading. It is no longer acceptable to just throw a shield layer onto the sensor structure to block display noise. Such an approach adds too much cost and thickness. Beyond displays, the prevalence of USBcharging connectors has made battery chargers into commodities, pulling every last cent from these devices. Capacitive-touchscreen ICs now sense picocoulombs of change in the presence of as much as 40V p-p ac noise. All of these factors add up to requirements for touchscreen ICs that are far more complex than what was required just last year. New innovations are needed, and so begin the noise wars. FEEDBACK OPTOCOUPLER AC RECTIFICATION OUTPUT RECTIFICATION POLAR- + INPUT CAPACITOR >9 F 1500 pF NONPOLAROUTPUT CAPACITOR OUT MICROCONTROLLER CIRCUIT NONPOLARINPUT CAPACITOR MODULATOR (PWM CONTROLLER) + POLARINPUT CAPACITOR >9 F TRANSFORMER FEEDBACK OPTOCOUPLER AC RECTIFICATION OUTPUT RECTIFICATION POLAR- + INPUT CAPACITOR OUT TRANSFORMER ≃220V NONPOLARINPUT CAPACITOR + POLARINPUT CAPACITOR MODULATOR (b) Figure 1 Flyback-converter chargers typically use PWM circuits (a), and low-cost, self-oscillating ringing-choke converters use a variant of the flyback design (b). www.edn-europe.com APRIL 2012 | EDN EuropE 49 http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE April 2012

Cover
Contents
International Rectifier
Contents
RS Components
Masthead
Comment
International Rectifier
Pulse
Analog Devices
FTDI
Digikey
Mouser
Baker's best
Digikey
Test & Measurement
Agilent Technologies
Digikey
Protect POE systems
Hirose
Coilcraft
Mesago
Cover Story
Digikey
Signal Integrity
RS Components insert
Balancing GBWP and quiescent current
Noise wars
Design Idea
Product Roundup
Tales from the cube

EDNE April 2012