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Cypress shows how to handle large touchscreens

What are the design considerations for large area touchscreens? Ask Todd Severson and Christi Juchmes

Cypress touchscreen

Each year, the screen size of touch-enabled consumer electronic devices continues to grow. Touchscreens gained popularity in smartphones and have quickly evolved into tablets.

As screen sizes increase, the main challenge for capacitive touch is maintaining the same performance users have come to expect from a mobile phone but over a larger screen. This means scanning more intersections, over more surface area, in the same amount of time.

In addition, the processor will have to work with smaller signals and more noise while trying to maintain speed, precision and responsiveness required for a desirable user interface experience.

Capacitive touchscreens operate by driving a transmit voltage into the sensor panel on your device that creates a signal charge. This signal is then received by the touchscreen controller, which is able to determine the sensor capacitance by measuring the change of the sensor charge.

The current received by the chip is equivalent to the capacitance of the panel multiplied by the voltage of the transmit drive (Q1 = C * VTX). A baseline circuit is able to remove the nominal non-touch sensor charge so the system can focus on measuring the change of sensor charge due to finger touch. This improves touch measurement, resolution and sensitivity.

The obstacle with larger screens is the transmit voltage has more surface area to cover and the resistance and capacitance of the sensor increases.

The touch panel is limited by the higher parasitic capacitance and resistance, affecting the RC time constant, which results in slower transmit frequency.

Transmit operating frequency affects signal settling, refresh rate and power consumption. The design aim is to determine the highest transmit operating frequency conditions for a consistent touch response across the panels while minimising scan time and power.

Most consumer electronics require a touch controller refresh rate of greater than 100Hz, or about 10ms. Certain applications, such as point of sale (POS) terminals, require even higher refresh rates to capture and recognise signatures and quick pen strokes.

It is challenging for large screens to maintain fast refresh rates because the touch controller needs to sweep greater surface area, gather data from all intersections, and process that data.

The two main components that effect refresh rate are how fast the screen is scanned and how fast the scanned data is processed.

A 17-inch screen has 11 times more intersections than a 5-inch screen with the same sensor characteristics (3108 vs. 275). So the 17-inch screen requires more scanning and processing power.

One technique is to make sure the touch controller has enough receive channels to sweep the screen in a single pass.

Most touchscreen stack-ups are composed of sensor patterns under the cover glass in an array of “unit cells” that run in the x- and y-direction, with x being transmit and y being receive or vice versa.

The receive channel will collect the data and use analogue-to-digital converters (ADCs) to convert the change in mutual capacitance of each unit cell into digital data for the host to interpret where the finger touch coordinates are located. If the number of receive channels or ADCs are inadequate, then it will take multiple scans and more time to sweep the entire panel.

This results in fewer samples that can be taken in a given time period, leading to an undesirable user experience.

A technique to help solve the processing problem is to add a bigger processor to the touch controller or offload some of the computing to the system’s main processing unit. This means sending capacitive data to the host-side and running algorithms on the applications or graphics processor.

One implementation would be to use the touchscreen controller to scan the sensor, search for first touch and then transfer the image to the host processor. The host would then process the full array, filter noise, find touch coordinates and track finger IDs.

The sensor on a touchscreen panel acts as a large antenna that is able to pick up system and environmental noise such as fluorescent lights, LCDs or chargers.

Larger screens act as larger antennas so it is easier to pick up noise and saturate a receive channel. This can greatly affect touch performance by causing false touches, dropped touches or a “locked-up” touchscreen that will not report data at all.

In order to overcome interference, the touchscreen controller needs to be able to increase signal or decrease noise.

Some of the primary ways to achieve a better signal-to-noise ratio (SNR) include boosting the transmit voltage to increase signal, using hardware and digital filtering to decrease noise or using frequency hopping to move away from noisy frequencies.

SNR increases linearly relative to transmit voltage. Transmit voltage can be delivered from a transmit charge pump or VDDA driver.

A charge pump is able to take a typical 2.7-3V power supply, found in most consumer electronic devices, and boost it up to a higher voltage.

The problem with large screens is that a charge pump has limited drive strength capability for high capacitance panels. This means that an external pump or power supply must be added, which can increase cost and power consumption.

If the signal is not enough, the other option is to minimise noise. Filters can be used to create a cleaner capacitive image.

If this is not effective the second line of defense is typically using frequency hopping to find a frequency where there is less interference. As mentioned earlier, large panels have higher parasitic capacitance and resistance, affecting the RC time constant that results in a slower transmit frequency.

A slower frequency means it is harder to scan the panel outside of the noise range. A higher transmit frequency gives the touch controller more room to move away from a noise source.

A maximum transmit frequency of 350kHz or greater is best, but a constant trade-off between SNR, refresh rate and power is required to optimise each device based on the customer’s objectives.

Power usually scales with larger screens due to the increased LCD size. A smart and energy efficient touchscreen controller would have multi-state power management and in each state has a unique scheme to lower power consumption such as an active state, low power state and deep sleep state. This is all managed by the touch controller’s configuration parameters.

Christi Juchmes is a product marketing specialist and Todd Severson is a product marketing engineer at Cypress Semiconductor



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