Touch-Screen Controller Works with Gloved Hands

At Mobile World Congress 2011, Maxim Integrated Products is demonstrating Max11855 and Max11871, projected capacitive touch-screen controllers offering four-point and 10-point touch detection. Building on the company's reputation for differentiated analogue and mixed-signal performance, Maxim's Tactouch family of capacitive touch-screen controllers addresses the various performance and price points demanded by OEMs. The latest Tactouch products offer superior sensitivity and noise immunity, making them the only devices capable of touch detection from a ballpoint/stylus and gloved hands.

These capabilities will enable a range of new user-interface experiences on handsets, tablets and other end equipment. The Max11871 is a mutual capacitance touch-screen controller capable of detecting and tracking up to 10-finger simultaneous touch. All processing is included on-chip to output up to 10 X,Y touch location coordinates, with a Z pressure metric associated to each touch point, via a standard I2C interface to a host microcontroller. The part's analogue front-end provides near 60dB signal-to-noise ratio (SNR) performance, equivalent to a 1000:1 ratio between touch and no-touch.

This is an order of magnitude 10x higher than other solutions on the market today. The SNR enables detection of very weak (in the femto-farad range) touch variations, such as from a hand waving near the screen (proximity detection), touch from a fine-tip stylus or ballpoint pen, or a hand covered by a glove. Also, it means that the touch point can be farther away from the sensor, enabling touch screens to have thicker cover glass or plastic for improved ruggedness. The Max11871 also includes a proprietary architecture to reject noise (by over 40dB) from external sources such as AC USB chargers, LCDs, or CFL lights with no additional external components.

This selectivity enables the part to be used in high LCD noise environments such as next-generation on-cell and in-cell touch screens. For applications only requiring four-point touch detection, the Max11855 and Max11856 implement a four-finger simultaneous mutual capacitive multi-touch solution, which eliminates any ghosting effect. As part of Maxim's Tactouch product line of haptics and touch-screen controllers, the Max11871 and Max11855/Max11856 can directly interface to the previously introduced Max11835/Max11836 haptic controllers/drivers for piezo actuators and electro-active polymers.

These high-voltage actuators mimic the feeling of a button click on a touch screen. The Max11871 comes in a 6 x 6mm 48-pin TQFN package with 32 cap-sense channels. A wafer-level package (WLP) option is planned for designs requiring an ultra-small footprint. The Max11855 comes in a 5 x 5mm 40-pin TQFN package with 25 cap-sense channels, typically sufficient for panels up to 4in.

The Max11856 provides 31 cap-sense channels in a 6 x 6mm 48-pin TQFN package, making it suitable for panels up to 7in depending on required touch accuracy. With on-chip clock oscillator and voltage references, all parts only require a few passives as external BOM. All parts are sampling now for qualified opportunities and mass production is scheduled to begin before the end of 2011's first quarter.

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