How to manage power in capacitive touch sensing designs: Part 1 - Embedded.com

How to manage power in capacitive touch sensing designs: Part 1

Capacitive touch sensing is replacing mechanical switches and push buttons in a wide variety of applications. Many battery-powered, handheld, and portable electronics have adopted capacitive touch sensing user interfaces.

The power constraints of these devices, along with the constant focus on energy efficiency, has made low-power design critical in capacitive touch sensing applications. Some best practices for reducing power consumption in capacitive touch sensing applications include:

.    Optimize the sensor parasitic capacitance (CP ) with board layout best practices
.    Use sleep mode and optimize the sensor report rate
.    Vary the report rate based on finger touch events
.    Use the priority rule to wake up from sleep mode

Optimize the sensor parasitic capacitance (CP ) with board layout best practices

Capacitive touch sensors typically consist of copper pads connected to a capacitive sensing controller input pins via traces. Figure 1 shows a typical capacitive sensor including electric field coupling lines.

Figure 1: Typical capacitive sensor pad construction

When a finger comes in contact with the overlay covering a sensor pad, it forms a simple parallel plate capacitor called finger capacitance (CF ). Even without a finger touching the overlay, the capacitive sensing controller measures some parasitic capacitance (CP ). CP is the sum of the distributed capacitance on the sensor. This includes the capacitance of the sensor pad due to its proximity to circuit ground, the trace connecting the capacitive sensing controller input pin and the sensor pad, vias, and the capacitive sensing controller input pin.

The capacitive sensing controller converts the capacitance measured at its input pin to digital counts using an A-to-D converter. The controller uses a DSP algorithm to continuously monitor the digital counts and identify increases in sensor capacitance due to a finger touch.

To accurately detect a finger touch, the A-to-D converter’s resolution has to be tuned to maintain a specific sensitivity level. If the CP is high, the resolution of A-to-D converter should be increased. Higher resolution results in a longer conversion time, which increases the average power consumption of a capacitive sensing application. To reduce power consumption, reduce the sensor(s) CP so that you can use a lower resolution A-to-D converter along with sleep mode.

The main components of CP are trace capacitance and sensor capacitance. CP is a nonlinear function of the annular gap between the sensor pad and ground, the distance between the trace and ground, trace length and width, and the sensor pad diameter.

There is no simple relationship between CP and PCB layout features but, in general, increasing the annular gap and decreasing the trace length and width will reduce CP . Unfortunately, widening the gap between the sensor pad and ground will decrease noise immunity. To achieve optimal CP and noise immunity, follow the capacitive sensing controller manufacturer’s PCB layout best practices.

Report rate and sleep mode worktogether to define how the capacitive touchsensors are sampled, as shown in Figure2 . Report rate defines howoften the sensors are sampled by the A-to-Dconverter. When a sensor is sampled, thecapacitive sensing controller is in activemode. When a sensor is not being sampled,the controller can be put into sleep mode.In sleep mode, the controller powers downall internal blocks and peripherals. Thismode is supported by most of the capacitivesensing controllers on the market.


Figure2: Sensor sampling with sleep mode
Clickon image to enlarge

Selecting a low report rate and a longersleep time are the keys to reducing theaverage power consumption. Report rate andsleep mode directly affect the capacitivesensing controller’s average currentconsumption, as defined in Equation1 .

Equation1: Average current calculation

Where:
IActive = Currentconsumed by the controller when itis sampling a sensor
TActive = Time it takesthe controller to sample all of thesensors
ISleep = Current consumedby the controller when it is insleep mode
TSleep = Amount of timethe controller remains in sleep mode

As a general rule of thumb, a humanfinger cannot touch a button fasterthan 150 ms. Typical A-to-Dconversion times range from 200—6000µs. This means that several A-to-Dconversions can be performed duringthe 150 ms a sensor is beingtouched. Optimize the report ratefor your specific A-to-D conversiontime.

Reducing CP helps toreduce TActive becausethe controller can use a lowerA-to-D converter resolution.Selecting a capacitive sensingcontroller with lower active currentreduces IActive . Keep inmind that sleep mode current has aminimal impact on the averagecurrent.

When selecting the report rate,consider how a user will interact with yourdevice. Consumer electronics may be poweredon for hours at a time without a fingertouching the user interface. For example,the buttons on an audio system or a TVremote are only used once in a while, whensomething like the volume or channel needsto be changed. However, once a button hasbeen touched, the user might touch multiplebuttons to get the intended result.

Based on this example, sensors can besampled less frequently (low report ratemode) until a button touch is detected, atwhich time the report rate can be increased(fast report rate mode) to enable quickresponse to subsequent touches, as shown inFigure 3 . If afinger touch is not detected for certainperiod of time, the capacitive sensingcontroller can revert back to low reportrate mode. Using a programmable capacitivesensing controller allows you to dynamicallychange the report rate based on touchevents.


Figure 3: Low vs. fast reportrate modes
Click on image to enlarge

To furtherreduce power consumption, user interfacebacklighting can be turned off while thecontroller is in low report rate mode.Once a touch is detected, the backlightingcan be turned back on while the controlleris in fast report rate mode.

Use the priority rule to wake upfrom sleep mode

In some applications all buttons shouldnot be able to wake the capacitive sensingcontroller from sleep mode.

For example, when a TV is turned off, thecontroller only needs to detect a fingertouch on the power button to turn the TVon. In this application, when the power isoff the controller only samples the powerbutton and does not sample any of theother buttons on the TV’s front panel.This reduces TActive , andsignificantly reduces average powerconsumption.

Let’s take a look at typical TV frontpanel:
.    Eight capacitive touchsensing buttons
.    Sampling time = 500 µsper button
.    IActive = 4mA
.    TActive =10 ms
.    ISleep = 1µA
.    TSleep = 90ms

Using Equation-1 ,when all buttons are sampled:
IAverage = 160 µA

When only the power button is sampled:
IAverage = 20 µA

In the second part of this article, I willdiscuss other methods of optimizingaverage power in capacitive touch sensingdesigns such as:

–    Using a gangedcapacitive sensor method to wake up fromsleep mode
–    Using a proximitysensor to wake up from sleep mode
–    Using an externalregulator to turn off power
–    Using the low poweroption specific to the capacitive sensingcontroller<

To read Part 2 in this series, go to “Ganged capacitive sensor model.

Vibheesh B is a senior applications engineerworking in Cypress Semiconductor's Consumer and Computation Division andhas specialized on Capacitive Touch Sensing applications since 2007.His responsibilities include defining technical requirements for newcapacitive sensing controllers, developing new capacitive sensingcontrollers, conducting system analysis, debugging technical issues forcustomers, technical writing, and failure-analysis debugging.

Joshan Abraham has been a product marketing engineer in the centralized marketing team at Cypress Semiconductor since early 2012. He is engaged in ramping up new products, driving new sales opportunities, and handling tactical aspects of the Capacitive Touch Sensing Business Unit (Capsense) of Cypress’ Programmable Systems Division

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