TSC2046E-Q1 Datasheet, PDF (22/28 Pages) Texas Instruments – LOW-VOLTAGE I/O TOUCH SCREEN CONTROLLER

TSC2046E-Q1
SBAS510 – JUNE 2010
www.ti.com
Layout
The following layout suggestions provide the most optimum performance from the TSC2046E. Many portable
applications, however, have conflicting requirements concerning power, cost, size, and weight. In general, most
portable devices have fairly clean power and grounds because most of the internal components are very low
power. This situation means less bypassing for the converter power and less concern regarding grounding. Still,
each situation is unique and the following suggestions should be reviewed carefully.
For optimum performance, care should be taken with the physical layout of the TSC2046E circuitry. The basic
SAR architecture is sensitive to glitches or sudden changes on the power supply, reference, ground connections,
and digital inputs that occur just prior to latching the output of the analog comparator. Therefore, during any
single conversion for an n-bit SAR converter, there are n windows in which large external transient voltages can
easily affect the conversion result. Such glitches can originate from switching power supplies, nearby digital logic,
and high-power devices. The degree of error in the digital output depends on the reference voltage, layout, and
the exact timing of the external event. The error can change if the external event changes in time with respect to
the DCLK input.
Because of the SAR architecture sensitvity, power to the TSC2046E should be clean and well bypassed. A 0.1µF
ceramic bypass capacitor should be placed as close to the device as possible. A 1µF to 10µF capacitor may also
be needed if the impedance of the connection between +V_CC or IOVDD and the power supplies is high.
Low-leakage capacitors should be used to minimize power dissipation through the bypass capacitors when the
TSC2046E is in power-down mode.
A bypass capacitor is generally not needed on the V_REF pin because the internal reference is buffered by an
internal op amp. If an external reference voltage originates from an op amp, make sure that it can drive any
bypass capacitor that is used without oscillation.
The TSC2046E architecture offers no inherent rejection of noise or voltage variation in regards to using an
external reference input. This is of particular concern when the reference input is tied to the power supply. Any
noise and ripple from the supply appears directly in the digital results. Whereas high-frequency noise can be
filtered out, voltage variation bacause of line frequency (50Hz or 60Hz) can be difficult to remove.
The GND pin must be connected to a clean ground point. In many cases, this is the analog ground. Avoid
connections which are too near the grounding point of a microcontroller or digital signal processor. If needed, run
a ground trace directly from the converter to the power-supply entry or battery connection point. The ideal layout
includes an analog ground plane dedicated to the converter and associated analog circuitry.
In the specific case of use with a resistive touch screen, care should be taken with the connection between the
converter and the touch screen. Although resistive touch screens have fairly low resistance, the interconnection
should be as short and robust as possible. Longer connections are a source of error, much like the on-resistance
of the internal switches. Likewise, loose connections can be a source of error when the contact resistance
changes with flexing or vibrations.
As indicated previously, noise can be a major source of error in touch screen applications (such as in
applications that require a backlit LCD panel). This EMI noise can be coupled through the LCD panel to the touch
screen and cause flickering of the converted data. Several things can be done to reduce this error; for instance,
using a touch screen with a bottom-side metal layer connected to ground to shunt the majority of noise to
ground. Additionally, filtering capacitors from Y+, Y-, X+, and X- pins to ground can also help. Caution should be
observed under these circumstances for settling time of the touch screen, especially operating in the
single-ended mode and at high data rates.
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