QT100A-ISG

 QT100A CHARGE-TRANSFER QTOUCH™ IC The QT100A charge-transfer (‘QT’) touch sensor is a self-contained digital IC capable of detecting near-proximity or touch. It will project a touch or proximity field through any dielectric like glass, plastic, stone, ceramic, and even most kinds of wood. It can also turn small metal-bearing objects into intrinsic sensors, making them responsive to proximity or touch. This capability, coupled with its ability to self-calibrate, can lead to entirely new product concepts. It is designed specifically for human interfaces, like control panels, appliances, toys, lighting controls, or anywhere a mechanical switch or button may be found. OUT 1 VSS 2 SNSK 3 SYNC 5 VDD 4 SNS 6-WSON Package This device can also project a proximity field to severa centimeters with the proper electrode and circuit design. These devices are intended to replace the QT100. Two package types are offered, one of which can directly replace the QT100’s SOT-23-6 package but with a reduced thickness. QT100A 6 SNSK 1 10 OUT SNS 2 9 N/C N/C 3 N/C 4 7 VSS 5 6 VDD AT A GLANCE QT100A 8 SYNC N/C 10-MSOP Package Number of keys: One Technology: Patented spread-spectrum charge-transfer. Key outline sizes: 6mm x 6mm or larger (panel thickness dependent); widely different sizes and shapes possible Electrode design: Solid or ring electrode shapes PCB Layers required: One Electrode materials: Etched copper, silver, carbon, Indium Tin Oxide (ITO) Electrode Substrates: PCB, FPCB, plastic films, glass Panel materials: Plastic, glass, composites, painted surfaces (low particle density metallic paints possible) Panel thickness: Up to 50mm glass, 20mm plastic (electrode size and Cs dependent) Key sensitivity: Settable via capacitor (Cs) Interface: Digital output, active high Moisture tolerance: Good Power: 2V ~ 5.5V; 6.5uA @ 2.0V typical Package: 10 pin MSOP and 6 pin WSON; both are RoHS compliant Signal processing: Self-calibration, auto drift compensation, noise filtering Applications: Control panels, consumer appliances, toys, lighting controls, mechanical switch or button Patents: QTouch™ (patented Charge-transfer method)  Copyright © 2008 QRG Ltd. QT100A_1R7.01_0308 Table 1.1 Pinlist Name SNSK SNS N/C N/C VSS VDD N/C SYNC N/C OUT Pin Pin 10MSOP 6WSON 1 3 2 4 3 4 5 2 6 5 7 -† 8 6‡ 9 10 1 Pin Type I I/O O Pwr Type Function Notes If Unused I/O I/O O O Pwr Pwr I I O O Sense pin Sense pin Unused Unused Ground Power Unused Sync & Mode Input Unused Output state To Cs + Key To Cs Leave open - do not connect Leave open - do not connect Always connect to Sync Leave open - do not connect - Ground +2.0 ~ +5.5V CMOS input only CMOS I/O CMOS push-pull output Power / ground Notes † In 6-WSON package this pin is internally connected to SYNC pin ‡ Pin is either Sync or Slow/Fast Mode depending on how strapped; see Section 2.1.  2 Copyright © 2008 QRG Ltd. QT100A_1R7.01_0308 Figure 1.1 Basic Circuit Configuration 1 Overview 2 ~ 5V 1.1 Introduction The QT100A is a digital burst mode charge-transfer (QT) sensor designed specifically for touch controls; it includes all hardware and signal processing functions necessary to provide stable sensing under a wide variety of changing conditions. Only a single low cost, noncritical capacitor is required for operation. SENSE ELECTRODE QT100A, 6-WSON Pkg. *Cb VDD SNSK OUT SNS 3 4 Rs Cs Cx SYNC This device is designed to replace the QT100. VSS 1.1.1 WSON package differences with QT100 A 6-pin WSON package version is available which uses the same basic PCB footprint as the QT100’s SOT-23-6 package. The 6-WSON package has a reduced package thickness. *Note: Bypass capacitor Cb should be tightly wired between Vdd and Vss The traces and any components associated with SNS and SNSK will become touch sensitive and should be treated with caution to limit the touch area to the desired location. In converting an existing design from the QT100 to the QT100A, it should be noted that the 6-WSON package has a bottom ‘center pad’ which must be either grounded (connected to Vss) or isolated. This center pad is wide enough that it can possibly touch the copper pads of the QT100’s SOT-23-6 footprint, potentially creating a short circuit between two or more pads. In order to prevent this the solder mask layer of the PCB under the device should be widened slightly, or, the copper PCB pads narrowed slightly to avoid contact with the center pad. The pinout and pad placement however are the same as for the QT100. A series resistor, Rs, should be placed in line with SNSK to the electrode to suppress ESD and EMC effects. 1.4 Sensitivity 1.4.1 Introduction The sensitivity on the QT100A is a function of things like the value of Cs, electrode size and capacitance, electrode shape and orientation, the composition and aspect of the object to be sensed, the thickness and composition of any overlaying panel material, and the degree of ground coupling of both sensor and object. An application note is available to assist in this conversion process. A 10-pin MSOP package version is also available. 1.4.2 Increasing Sensitivity 1.1.2 Electrical differences with QT100 In some cases it may be desirable to increase sensitivity; for example, when using the sensor with very thick panels having a low dielectric constant. Sensitivity can often be increased by using a larger electrode or reducing panel thickness. Increasing electrode size can have diminishing returns, as high values of Cx will reduce sensor gain. The QT100A is behaviorally identical to the QT100, and no change in the value of Cs is required to achieve the same sensitivity as the QT100. 1.2 Basic Operation Figure 1.1 shows a basic circuit using the device. The value of Cs also has a dramatic effect on sensitivity, and this can be increased in value with the trade-off of slower response time and more power. Increasing the electrode's surface area will not substantially increase touch sensitivity if its diameter is already much larger in surface area than the object being detected. Panel material can also be changed to one having a higher dielectric constant, which will better help to propagate the field. The QT100A employs bursts of charge-transfer cycles to acquire its signal. Burst mode permits power consumption in the microamp range, dramatically reduces RF emissions, lowers susceptibility to EMI, and yet permits excellent response time. Internally the signals are digitally processed to reject impulse noise, using a 'consensus' filter which requires four consecutive confirmations of a detection before the output is activated. Ground planes around and under the electrode and its SNSK trace will cause high Cx loading and destroy gain. The possible signal-to-noise ratio benefits of ground area are more than negated by the decreased gain from the circuit, and so ground areas around electrodes are discouraged. Metal areas near the electrode will reduce the field strength and increase Cx loading and should be avoided, if possible. Keep ground away from the electrodes and traces. The QT switches and charge measurement hardware functions are all internal to the QT100A. 1.3 Electrode Drive For optimum noise immunity, the electrode should only be connected to SNSK. In all cases the rule Cs>>Cx must be observed for proper operation; a typical load capacitance (Cx) ranges from 2-20pF while Cs is usually from 2-50nF. 1.4.3 Decreasing Sensitivity In some cases the QT100A may be too sensitive. In this case gain can be easily lowered by decreasing Cs. Increasing amounts of Cx destroy gain, therefore it is important to limit the amount of stray capacitance on both SNS terminals. This can be done, for example, by minimizing trace lengths and widths and keeping these traces away from power or ground traces or copper pours.  3 Copyright © 2008 QRG Ltd. QT100A_1R7.01_0308 2.1.2 Fast Mode (SYNC = 1) Key touch Figure 2.1 Low Power Mode ~85ms SNSK QT100A sleep The QT100A runs in Fast mode if the SYNC pin is permanently high. In this mode the QT100A runs at maximum speed at the expense of increased current consumption. Fast mode is useful when speed of response is the prime design requirement. The delay between bursts in Fast mode is approximately 1ms, as shown in Figure 2.2. fast detect integrator sleep sleep 2.1.3 Low Power Mode (SYNC = 0) The QT100A runs in Low Power (LP) mode if the SYNC pin is held low. In this mode it sleeps for approximately 85ms at the end of each burst, saving power but slowing response. On detecting a possible key touch, it temporarily switches to Fast mode until either the key touch is confirmed or found to be spurious (via the detect integration process). It then returns to LP mode after the key touch is resolved as shown in Figure 2.1. SYNC OUT (SYNC held low) Figure 2.2 Fast Mode Bursts (SYNC held high) 2.1.4 Sync Mode It is possible to synchronize the device to an external clock source by placing an appropriate waveform on the SYNC pin. Sync mode can synchronize multiple QT100A devices to each other to prevent cross-interference, or it can be used to enhance noise immunity from low frequency sources such as 50Hz or 60Hz mains signals. SNSK QT100A ~1ms SYNC The Sync pin is sampled at the end of each burst. If the device is in Fast mode and the Sync pin is sampled high, then the device continues to operate in Fast mode (Figure 2.2). If SYNC is sampled low, then the device goes to sleep. From then on, it will operate in Sync mode (Figure 2.1). Therefore, to guarantee entry into Sync mode the low period of the Sync signal should be longer than the burst length (Figure 2.3). Figure 2.3 Sync Mode (triggered by negative edges on SYNC) SNSK QT100A sleep sleep SYNC sleep Revert to Fast Mode However, once Sync mode has been entered, if the Sync signal consists of a series of short pulses (>10µs) then a burst will only occur on the falling edge of each pulse (Figure2.4). slow mode sleep period SNSK QT100A sleep sleep sleep In Sync mode, the device will sleep after each measurement burst (just as in LP mode) but will be awakened by the falling edge of the Sync signal, resulting in a new measurement burst. If Sync remains unchanged for a period longer than the LP mode sleep period (about 85ms), the device will resume operation in either Fast or LP mode depending on the level of the Sync pin (Figure 2.3). Revert to Slow Mode slow mode sleep period SYNC There is no DI in Sync mode (each touch is a detection) but the Max On-duration will depend on the time between Sync pulses; see Sections 2.3 and 2.4. Recalibration timeout is a fixed number of measurements so will vary with the Sync period. Figure 2.4 Sync Mode (Short Pulses) SNSK QT100A >10us >10us >10us 2.2 Threshold SYNC The internal signal threshold level is fixed at 13 counts of change with respect to the internal reference level, which in turn adjusts itself slowly in accordance with the drift compensation mechanism. 2 Operation Specifics The QT100A employs a hysteresis dropout of 3 counts of the delta between the reference and threshold levels. 2.1 Run Modes 2.3 Max On-duration 2.1.1 Introduction If an object or material obstructs the sense pad the signal may rise enough to create a detection, preventing further operation. To prevent this, the sensor includes a timer which monitors detections. If a detection exceeds the timer setting the sensor performs a full recalibration. This is known as the The QT100A has three running modes which depend on the logic level applied to the SYNC pin.  4 Copyright © 2008 QRG Ltd. QT100A_1R7.01_0308 Max On-duration feature and is set to ~80s (at 3V). This will vary slightly with Cs and if Sync mode is used. As the internal timebase for Max On-duration is determined by the burst rate, the use of Sync can cause dramatic changes in this parameter depending on the Sync pulse spacing. Figure 2.5 Drift Compensation S ignal H ysteresis Threshold 2.4 Detect Integrator R eference It is desirable to suppress detections generated by electrical noise or from quick brushes with an object. To accomplish this, the QT100A incorporates a ‘detect integration’ (DI) counter that increments with each detection until a limit is reached, after which the output is activated. If no detection is sensed prior to the final count, the counter is reset immediately to zero. In the QT100A, the required count is four. In LP mode the device will switch to Fast mode temporarily in order to resolve the detection more quickly; after a touch is either confirmed or denied the device will revert back to normal LP mode operation automatically. Output Once an object is sensed, the drift compensation mechanism ceases since the signal is legitimately high, and therefore should not cause the reference level to change. The DI can also be viewed as a 'consensus' filter, that requires four successive detections to create an output. The QT100A's drift compensation is 'asymmetric'; the reference level drift-compensates in one direction faster than it does in the other. Specifically, it compensates faster for decreasing signals than for increasing signals. Increasing signals should not be compensated for quickly, since an approaching finger could be compensated for partially or entirely before even approaching the sense electrode. However, an obstruction over the sense pad, for which the sensor has already made full allowance, could suddenly be removed leaving the sensor with an artificially elevated reference level and thus become insensitive to touch. In this latter case, the sensor will compensate for the object's removal very quickly, usually in only a few seconds. 2.5 Forced Sensor Recalibration The QT100A has no recalibration pin; a forced recalibration is accomplished when the device is powered up or after the recalibration timeout. However, supply drain is low so it is a simple matter to treat the entire IC as a controllable load; driving the QT100A's Vdd pin directly from another logic gate or a microcontroller port will serve as both power and 'forced recal'. The source resistance of most CMOS gates and microcontrollers are low enough to provide direct power without problem. 2.6 Drift Compensation With large values of Cs and small values of Cx, drift compensation will appear to operate more slowly than with the converse. Note that the positive and negative drift compensation rates are different. Signal drift can occur because of changes in Cx and Cs over time. It is crucial that drift be compensated for, otherwise false detections, nondetections, and sensitivity shifts will follow. Drift compensation (Figure 2.5). is performed by making the reference level track the raw signal at a slow rate, but only while there is no detection in effect. The rate of adjustment must be performed slowly, otherwise legitimate detections could be ignored. The QT100A drift compensates using a slew-rate limited change to the reference level; the threshold and hysteresis values are slaved to this reference. 2.7 Response Time The QT100A's response time is highly dependent on run mode and burst length, which in turn is dependent on Cs and Cx. With increasing Cs, response time slows, while increasing levels of Cx reduce response time. The response time will also be a lot slower in LP or Sync mode due to a longer time between burst measurements. Figure 2.6 Figure 2.7 Getting HeartBeat pulses with a pull-up resistor Using a micro to obtain HeartBeat pulses in either output state VDD Ro 5 VDD 1 3 SNSK OUT R0 Microcontroller HeartBeat™ Pulses SNS 4 OUT SNSK SNS Port_M.y SYNC 1 Port_M.x SYNC 3 4 6 6 VSS 2  5 Copyright © 2008 QRG Ltd. QT100A_1R7.01_0308 2.8 Spread Spectrum 3.1 Sample Capacitor The QT100A modulates its internal oscillator by ±7.5percent during the measurement burst. This spreads the generated noise over a wider band reducing emission levels. This also reduces susceptibility since there is no longer a single fundamental burst frequency. Cs is the charge sensing sample capacitor. The required Cs value depends on the thickness of the panel and its dielectric constant. Thicker panels require larger values of Cs. Typical values are 2nF to 50nF depending on the sensitivity required; larger values of Cs demand higher stability and better dielectric to ensure reliable sensing. 2.9 Output Features The Cs capacitor should be a stable type, such as X7R ceramic or PPS film. For more consistent sensing from unit to unit, 5percent tolerance capacitors are recommended. X7R ceramic types can be obtained in 5percent tolerance at little or no extra cost. In applications where high sensitivity (long burst length) is required the use of PPS capacitors is recommended. 2.9.1 Output The output of the QT100A is active-high upon detection. The output will remain active-high for the duration of the detection, or until the Max On-duration expires, whichever occurs first. If a Max On-duration timeout occurs first, the sensor performs a full recalibration and the output becomes inactive (low) until the next detection. 3.2 Power Supply, PCB Layout The power supply can range between 2.0V and 5.5V. At 3V current drain averages less than 500µA in Fast mode. 2.9.2 HeartBeat™ Output The QT100A output has a HeartBeat™ ‘health’ indicator superimposed on it in all modes. This operates by taking the output pin into a three-state mode for 15µs once before every QT burst. This output state can be used to determine that the sensor is operating properly, or it can be ignored, using one of several simple methods. If the power supply is shared with another electronic system, care should be taken to ensure that the supply is free of digital spikes, sags, and surges which can adversely affect the QT100A. The QT100A will track slow changes in Vdd, but it can be badly affected by rapid voltage fluctuations. It is highly recommended that a separate voltage regulator be used just for the QT100A to isolate it from power supply shifts caused by other components. The HeartBeat indicator can be sampled by using a pull-up resistor on the OUT pin, and feeding the resulting positive-going pulse into a counter, flip flop, one-shot, or other circuit. The pulses will only be visible when the chip is not detecting a touch. If desired, the supply can be regulated using a Low Dropout (LDO) regulator, although such regulators often have poor transient line and load stability. See Application Note AN-KD02 for further information on power supply considerations. If the sensor is wired to a microcontroller as shown in Figure2.7, the microcontroller can reconfigure the load resistor to either Vss or Vdd depending on the output state of the QT100A, so that the pulses are evident in either state. Parts placement: The chip should be placed to minimize the SNSK trace length to reduce low frequency pickup, and to reduce stray Cx which degrades gain. The Cs and Rs resistors (see Figure 1.1) should be placed as close to the body of the chip as possible so that the trace between Rs and the SNSK pin is very short, thereby reducing the antenna-like ability of this trace to pick up high frequency signals and feed them directly into the chip. A ground plane can be used under the chip and the associated discretes, but the trace from the Rs resistor and the electrode should not run near ground to reduce loading. Electromechanical devices like relays will usually ignore the short Heartbeat pulse. The pulse also has too low a duty cycle to visibly affect LEDs. It can be filtered completely if desired, by adding an RC filter to the output, or if interfacing directly and only to a high-impedance CMOS input, by doing nothing or at most adding a small noncritical capacitor from OUT to Vss. 2.9.3 Output Drive The OUT pin is active high and can sink or source up to 2mA. When a large value of Cs (>20nF) is used the OUT current should be limited to