TSC2008-Q1 Datasheet, PDF (32/39 Pages) Texas Instruments – 1.2V to 3.6V, 12-Bit, Nanopower, 4-Wire Micro TOUCH SCREEN CONTROLLER with SPI™

TSC2008-Q1
SBAS552 – JUNE 2011
LAYOUT
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The following layout suggestions should obtain optimum performance from the TSC2008-Q1. Keep in mind that
many portable applications have conflicting requirements for 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 would mean less bypassing for the converter power and less concern regarding grounding.
However, 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 TSC2008-Q1 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 before 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 might 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
SCLK input.
With this in mind, power to the TSC2008-Q1 should be clean and well-bypassed. A 0.1μF ceramic bypass
capacitor should be placed as close to the device as possible. In addition, a 1μF to 10μF capacitor may also be
needed if the impedance of the connection between VDD/REF and the power supply is high.
A bypass capacitor is generally not needed on the VDD/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 TSC2008-Q1 architecture offers no inherent rejection of noise or voltage variation with regard to using an
external reference input, which is of particular concern when the reference input is tied to the power supply. Any
noise and ripple from the supply appear directly in the digital results. While high-frequency noise can be filtered
out, voltage variation as a result of line frequency (50Hz or 60Hz) can be difficult to remove. Some package
options have pins labeled as VOID. Avoid any active trace going under any pin marked as VOID unless it is
shielded by a ground or power plane.
The GND pin should be connected to a clean ground point. In many cases, this point is the analog ground. Avoid
connections that 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. Because resistive touch screens have fairly low resistance, the interconnection
should be as short and robust as possible. 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 (for example,
applications that require a back-lit LCD panel). This electromagnetic interference (EMI) noise can be coupled
through the LCD panel to the touch screen and cause flickering of the converted A/D converter data. Several
things can be done to reduce this error, such as using a touch screen with a bottom-side metal layer connected
to ground, which couples the majority of noise to ground. Additionally, filtering capacitors, from Y+, Y–, X+, and
X– to ground, can also help. Note, however, that the use of these capacitors increases screen settling time and
requires a longer time for panel voltages to stabilize. The resistor value varies depending on the touch screen
sensor used. The PENIRQ pull-up resistor (R_IRQ) may be adequate for most of sensors. If not used, the
general-purpose analog input to the converter (AUX) should be connected to the analog ground plane.
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