Low Voltage Controller for Touch Screens

The AD7879 / AD7889 are 12-bit successive approximation analog-to-digital converters (SAR ADCs) with a synchronous serial interface and low on-resistance switches for driving 4-wire resistive touch screens. The AD7879/AD7889 work with a very low power supply—a single 1.6 V to 3.6 V supply—and feature a throughput rate of 105 kSPS. The devices include a shutdown mode that reduces current consumption to less than 6 μA.

To reduce the effects of noise from LCDs and other sources, the AD7879 / AD7889 contain a preprocessing block. The preprocessing function consists of a median filter and an averaging filter. The combination of these two filters provides a more robust solution, discarding the spurious noise in the signal and keeping only the data of interest. The size of both filters is programmable. Other user-programmable conversion controls include variable acquisition time and first conversion delay; up to 16 averages can be taken per conversion. The AD7879 / AD7889 can run in slave mode or standalone (master) mode, using an automatic conversion sequencer and timer.

The AD7879 / AD7889 have a programmable pin that can operate as an auxiliary input to the ADCs, as a battery monitor, or as a general-purpose input/output (GPIO). In addition, a programmable interrupt output can operate in three modes: as a general-purpose interrupt to signal when new data is available (DAV), as an interrupt to indicate when limits are exceeded (INT), or as a pen-down interrupt when the screen is touched (PENIRQ). The AD7879/AD7889 offer temperature measurement and touch pressure measurement.

The AD7879 is available in a 12-ball, 1.6 mm × 2 mm WLCSP and in a 16-lead, 4 mm × 4 mm LFCSP. The AD7889 is available in a backside coated version of the WLCSP. Both devices support an SPI interface (AD7879 / AD7889) or an I2C interface (AD7879-1 / AD7889-1).

APPLICATIONS

* Personal digital assistants

* Smart handheld devices

* Touch screen monitors

* Point-of-sale terminals

* Medical devices

* Cell phones

* 4-wire touch screen interface

* 1.6 V to 3.6 V operation

* Median and averaging filter to reduce noise

* Automatic conversion sequencer and timer

* User-programmable conversion parameters

* Auxiliary analog input/battery monitor (0.5 V to 5 V)

* 1 optional GPIO