Capacitive touchscreens in consumer electronics to took off with the launch of Apple’s iPhone in 2007. The 3.5” screen introduced a multi-touch user experience that changed the way we interact with our electronics. Touchscreen displays are now a standard in consumer electronic products such as DSCs (Digital Still Cameras), PNDs (Portable Navigation Devices), e-readers, tablets, Ultrabooks and AIO (All-In-One) PCs.
A key trend in all of these devices is the move to larger screen sizes. Not only are capacitive touchscreens growing to address new market segments such as Ultrabooks or notebooks, they are also increasing within their current product segment. For example, smartphone OEMs are making the move from smartphones to superphones, providing larger screen sizes as a key differentiation in the market.
The main product segments for touch-enabled devices today are smartphones with screen sizes between 3” to 5”; super-phone or phablet in the range of 5” to 8”; tablets 8” to 11.6;, Ultrabooks 11.6” to 15.6”; and notebooks ranging as high as 17”. Tablets are considered one of the fastest ramping mobile devices in its five years of product history; sales are predicted to overtake PC sales by 2015 (Figure 1 ). This is causing PC vendors to shift their focus to adopting touch-friendly designs such as convertible notebooks that can function as notebooks or tablets.
As screen sizes of touch-enabled devices grow larger, the main challenge for designers is maintaining the same high performance users have come to expect from a cell 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 has to work with less signal and more noise while still maintaining the speed, precision, and responsiveness required for a desirable user interface experience.
Users expect large screen devices to have similar performance and touch experience to that of their smartphones, but large screen devices often deal with different use cases than what is typical on a smaller phone. Notebooks or PCs are more likely to be used while plugged into a power source, there is more surface area to rest palms or other large objects on the screen when typing, and users are more likely to set larger devices on a table or in their lap instead of holding it in their hands.
All of these conditions and circumstances change the electrical properties of a device. The key ingredients to a robust and responsive user experience include sensitivity, tracking multiple moving touch objects, recognizing and tracking fingers in different noise environments, recognizing and tracking fingers under different environmental conditions, and maintaining acceptable power consumption to achieve the desired battery life.
Capacitive touchscreens operate by driving a transmit voltage into the sensor panel on the 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 main problem with larger screens is that 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. The transmit operating frequency affects signal settling, refresh rate and power consumption. The goal is to determine the highest transmit operating frequency conditions for a consistent touch response across the panels while minimizing scan time and power.
Refresh rates versus user interface needs
Refresh rate is the number of times in a second that the touchscreen controller can measure a touch on the screen and report it back to the host processor. A higher refresh rate will provide a responsive user experience by collecting more x/y data coordinates in a shorter amount of time. Most consumer electronics devices require a touch controller refresh rate of greater than 100 Hz, or about 10 ms. Certain applications, such as digital drawing pads or Point of Sale (POS) terminals require even higher refresh rates to capture and recognize 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 the intersections, and then 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” screen has 11 times more intersections than a 5” screen with the same sensor characteristics (3108 vs. 275). In order to maintain the user experience of the 5” screen, the 17” screen requires more scanning and processing power.
One technique to help solve the scanning problem 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 analog to digital converters (ADC) 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 unsatisfactory 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 will then process the full array, filter noise, find touch coordinates and track finger IDs. This use of parallel processing allows the heavy number crunching to be done in the multi-GHz, multi-core processors that serve as a host for the touchscreen and display.Changing requirements for panel SNR
SNR (signal to noiseratio) is the ratio of signal power to noise power, or, in other words,the ratio of useful information to false or irrelevant data. The sensoron a touchscreen panel acts as a large antenna (Figure 2 ) that is able to pick up system and environmental noise such as fluorescent lights, LCDs or chargers.
Largerscreens act as larger antennas so it is easier to pick up noise andsaturate a receive channel. This can greatly affect touch performance bycausing false touches, dropped touches, or a locked up touchscreen thatwill not report data at all. In order to overcome this interference,the touchscreen controller needs to be able to increase signal ordecrease noise. Some of the primary ways to achieve better SNR includeboosting the transmit voltage to increase signal, using hardware anddigital filtering to decrease noise, or using frequency hopping to moveaway from noisy frequencies.
SNR increases linearly,proportional with transmit voltage. Transmit voltage can be deliveredfrom a transmit charge pump or VDDA driver. A charge pump is able totake a typical 2.7-3V power supply, found in most consumer electronicdevices, and boost it up to a higher voltage. The problem with largescreens is that a charge pump has limited drive strength capability forhigh capacitance panels. This means that an external pump or powersupply must be added, which can increase cost and power consumption.
Ifthere is not enough signal, the other option is to minimize noise. Thefirst line of defense is using filters to create a cleaner capacitiveimage. If this is not effective the second line of defense is usingfrequency hopping to find a frequency where there is less interference.
Asmentioned earlier, large panels have higher parasitic capacitance andresistance, affecting the RC time constant that results in a slowertransmit frequency. A slower frequency means it is harder to scan thepanel outside of the noise range. A higher transmit frequency gives thetouch controller more room to move away from a noise source. A maxtransmit frequency of 350 kHz or greater is ideal, but a constanttrade-off between SNR, refresh rate and power is required to optimizeeach device based on the customer’s objectives. An individual playinggames on a desktop PC is more interested in responsiveness than powerconsumption, whereas portable devices need to account for powerconsumption to save on battery life.
Bigger screens and power consumption
Asmobility becomes a bigger part of our lives, power consumption is a keyfactor in a consumer’s selection for portable electronic devices.Market surveys (Figure 3 ) show that a majority of users believebattery life is one of the most important features when purchasing a newportable device.
TheLCD is a big portion of the power draw from the overall system. Powerusually scales with larger screens due to the increased LCD size. Oneway of maintaining battery life is to put a larger battery pack in thesystem. However, this increases the weight of the system and affects theuser experience in terms of portability. Another alternative is todecrease performance by reducing refresh rate, reducing transmitvoltage, disabling various digital filters, or using the lowest possibleanalog and digital power supplies. Again, these solutions negativelyimpact the user experience so they are not ideal options.
Asweight and performance are key factors to a good device, the bestresolution for extending battery life is to optimize power draw forindividual components in the system. From a touchscreen controller pointof view, that means having flexible power management schemes for thedevice.
The overall power consumption depends on the state or usage of the device (Figure 4 ).A smart and energy efficient touchscreen controller has multi-statepower management in which each state has a unique scheme to lower powerconsumption, such as an active state, low power state, and deep sleepstate. This is all managed by the touch controller’s configurationparameters.
- The active state provides the fastest touch response time because the touchscreen is actively scanned to determine the presence of a touch and identify the coordinates.
- The low power state is entered when no touch is detected after a certain time during the active state. This state further reduces power with corresponding increase in the response time. Any touch detected will automatically switch the device into active state.
- The deep sleep state has the lowest power consumption. No scanning is performed and no touches are reported. An interrupt is required to wake up the touch screen controller and put it into active state.
Thevarious power states are determined by the system environment. Forexample, if the screen hasn’t been touched in a while, the system willdeactivate the user interface to save battery life. This is done by thehost managing the components in the device, for example by turning offthe LCD screen and placing the touch controller into a low-power state.When a touch is detected in the low-power state, the touchscreencontroller will transition to active mode and continue scanning todetermine the touch coordinates on the panel. If no touch is detected inthe low-power mode, the host will drive the touch controller into deepsleep to conserve power. These dynamic power management states provideconsumers flexibility between touch performance and power consumptionfor mobile devices on-the-go.
Maintaining satisfactory userexperience as touchscreens grow takes a system wide approach.Touchscreens are limited by physics, and if capacitive touch is toremain the technology of choice in mobile consumer electronic devices,then ingenuity and integration are key. New touchscreen materials arebeing developed to increase panel speeds, and host processingarchitectures are being defined to offload some of the heavy numbercrunching. Hardware and software improvements are constantly being madeto increase signal strength while filtering out noise. A system wideapproach to power consumption is being used to increase battery life.Making this all more cost effective is the next big challenge fordesigners.
Todd Severson is a Product MarketingEngineer for TrueTouch touchscreen solutions at Cypress SemiconductorCorp. He has a BS degree in Engineering Management with a concentrationin Mechanical Engineering from the United States Military Academy. Youmay reach him at
Henry Wong is a Senior Product Marketing Manager for TrueTouch touchscreensolutions at Cypress. He has a BS degree in Computer and SystemsEngineering from Rensselaer Polytechnic Institute. Henry has over 16years of experience in engineering and marketing experience in thesemiconductor and consumer electronics industry worldwide. You may reachhim at .