AT42QT1011

Features • Number of Keys: – One – Configurable as either a single key or a proximity sensor Technology: – Patented spread-spectrum charge-transfer (direct mode) Key outline sizes: – 6 mm x 6 mm 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 12 mm glass, 6 mm plastic (electrode size and Cs dependent) Key sensitivity: – Settable via capacitor (Cs) Interface: – Digital output, active high Moisture tolerance: – Good Power: – 1.8V – 5.5V; 17 µA at 1.8V typical Package: – 6-pin SOT23-6 RoHS compliant Signal processing: – Self-calibration, auto drift compensation, noise filtering – Infinite max on-duration Applications: – Control panels, consumer appliances, proximity sensor applications, toys, lighting controls, mechanical switch or button, Patents: – QTouch® (patented charge-transfer method) – HeartBeat™ (monitors health of device) • • • • • • • One-channel Touch Sensor IC AT42QT1011 • • • • • • • • • 9542G–AT42–03/10 1. Pinout and Schematic 1.1 Pinout Configuration OUT VSS SNSK 1 2 3 6 5 4 SYNC/MODE VDD SNS 1.2 Pin Descriptions Pin Listing Pin 1 2 3 4 5 6 Type O P I/O I/O P I Comments Output state Supply ground Sense pin Sense pin Power SYNC and Mode Input – – Cs + Key Cs – Pin is either SYNC/Slow/Fast Mode, depending on logic level applied (see Section 3.1 on page 4) I/O P Input and output Ground or power If Unused, Connect To... Table 1-1. Name OUT Vss SNSK SNS Vdd SYNC I O Input only Output only, push-pull 1.3 Schematic Figure 1-1. Basic Circuit Configuration VDD 5 SENSE ELECTRODE 3 VDD 1 OUT Rs Cs SNSK SNS 4 SYNC/MODE 6 VSS 2 Cx Note: A bypass capacitor should be tightly wired between Vdd and Vss and kept close to pin 5. 2 AT42QT1011 9542G–AT42–03/10 AT42QT1011 2. Overview of the AT42QT1011 2.1 Introduction The AT42QT1011 (QT1011) is a digital burst mode charge-transfer (QT™) sensor that is capable of detecting near-proximity or touch, making it ideal for implementing touch controls. With the proper electrode and circuit design, the self-contained digital IC will project a touch or proximity field to several centimeters 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. The QT1011 is designed specifically for human interfaces, like control panels, appliances, toys, lighting controls, or anywhere a mechanical switch or button may be found. 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 capacitor is required for operation. 2.2 Basic Operation Figure 1-1 on page 2 shows a basic circuit. The QT1011 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. The QT switches and charge measurement hardware functions are all internal to the QT1011. 2.3 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 5-20 pF while Cs is usually about 2-50 nF. 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. 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. A series resistor, Rs, should be placed in line with SNSK to the electrode to suppress ESD and EMC effects. 2.4 2.4.1 Sensitivity Introduction The sensitivity on the QT1011 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. 3 9542G–AT42–03/10 2.4.2 Increasing Sensitivity 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, or when the device is used as a proximity sensor. 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 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. In the case of proximity detection, usually the object being detected is on an approaching hand, so a larger surface area can be effective. 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. 2.4.3 Decreasing Sensitivity In some cases the QT1011 may be too sensitive. In this case gain can be easily lowered further by decreasing Cs. Proximity Sensing By increasing the sensitivity, the QT1011 can be used as a very effective proximity sensor, allowing the presence of a nearby object (typically a hand) to be detected. In this scenario, as the object being sensed is typically a hand, very large electrode sizes can be used, which is extremely effective in increasing the sensitivity of the detector. In this case, the value of Cs will also need to be increased to ensure improved sensitivity, as mentioned in Section 2.4.2. Note that, although this affects the responsiveness of the sensor, it is less of an issue in proximity sensing applications; in such applications it is necessary to detect simply the presence of a large object, rather than a small, precise touch. 2.4.4 3. Operation Specifics 3.1 3.1.1 Run Modes Introduction The QT1011 has three running modes which depend on the state of the SYNC pin (high or low). 3.1.2 Fast Mode The QT1011 runs in Fast mode if the SYNC pin is permanently high. In this mode the QT1011 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 1 ms, as shown in Figure 3-1. 4 AT42QT1011 9542G–AT42–03/10 AT42QT1011 Figure 3-1. Fast Mode Bursts (SYNC Held High) SNSK ~1ms SYNC 3.1.3 Low Power Mode The QT1011 runs in Low Power (LP) mode if the SYNC pin is held low. In this mode it sleeps for approximately 80 ms 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 3-2 on page 5. Figure 3-2. Low Power Mode (SYNC Held Low) Key touch ~80 ms Fast detect integrator Sleep SNSK Sleep Sleep SYNC OUT 3.1.4 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 QT1011 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. 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 3-1 on page 5). If SYNC is sampled low, then the device goes to sleep. From then on, it will operate in SYNC mode (Figure 3-2). Therefore, to guarantee entry into SYNC mode the low period of the SYNC signal should be longer than the burst length (Figure 3-3 on page 6). 5 9542G–AT42–03/10 Figure 3-3. SYNC Mode (Triggered by SYNC Edges) SNSK sleep sleep sleep Revert to Fast Mode SYNC slow mode sleep period SNSK sleep sleep sleep slow mode sleep period Revert to Slow Mode SYNC However, once SYNC mode has been entered, if the SYNC signal consists of a series of short pulses (>10 µs) then a burst will only occur on the falling edge of each pulse (Figure 3-4 on page 6) instead of on each change of SYNC signal, as normal (Figure 3-3). In SYNC mode, the device will sleep after each measurement burst (just as in LP mode) but will be awakened by a change in the SYNC signal in either direction, resulting in a new measurement burst. If SYNC remains unchanged for a period longer than the LP mode sleep period (about 80 ms), the device will resume operation in either Fast or LP mode depending on the level of the SYNC pin (Figure 3-3). There is no detect integrator (DI) in SYNC mode (each touch is a detection); see Section 3.4. Recalibration timeout is a fixed number of measurements so will vary with the SYNC period. Figure 3-4. SYNC Mode (Short Pulses) SNSK >10us >10us >10us SYNC 3.2 Threshold The internal signal threshold level is fixed at 10 counts of change with respect to the internal reference level, which in turn adjusts itself slowly in accordance with the drift compensation mechanism. The QT1011 employs a hysteresis dropout of two counts of the delta between the reference and threshold levels. 3.3 Max On-duration The max on-duration of this device is infinite; that is, the device will not automatically recalibrate due to a persistant detection. 6 AT42QT1011 9542G–AT42–03/10 AT42QT1011 3.4 Detect Integrator It is desirable to suppress detections generated by electrical noise or from quick brushes with an object. To accomplish this, the QT1011 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 QT1011, 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. The DI can also be viewed as a “consensus filter” that requires four successive detections to create an output. 3.5 Forced Sensor Recalibration The QT1011 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 QT1011's Vdd pin directly from another logic gate or a microcontroller port will serve as both power and “forced recalibration”. The source resistance of most CMOS gates and microcontrollers is low enough to provide direct power without problem. 3.6 Drift Compensation 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 3-5) 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 QT1011 drift compensates using a slew-rate limited change to the reference level; the threshold and hysteresis values are slaved to this reference. 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. Figure 3-5. Drift Compensation S ignal H ysteresis Threshold R eference Output 7 9542G–AT42–03/10 The QT1011'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. 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. 3.7 Response Time The QT1011'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. 3.8 Spread Spectrum The QT1011 modulates its internal oscillator by ±7.5 percent 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. 3.9 3.9.1 Output Features Output The output of the QT1011 is active-high upon detection. The output will remain active-high for the duration of the detection. 3.9.2 HeartBeat™ Output The QT1011 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 15 µs, once before every QT burst. This output state can be used to determine that the sensor is operating properly, using one of several simple methods, or it can be ignored. The HeartBeat indicator can be sampled by using a pull-up resistor on the OUT pin (Figure 3-6), 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. 8 AT42QT1011 9542G–AT42–03/10 AT42QT1011 Figure 3-6. Obtaining HeartBeat Pulses with a Pull-up Resistor HeartBeat™ Pulses VDD 5 Ro 1 VDD OUT SNSK 3 4 6 SNS SYNC VSS 2 If the sensor is wired to a microcontroller as shown in Figure 3-7 on page 9, the microcontroller can reconfigure the load resistor to either Vss or Vdd depending on the output state of the QT1011, so that the pulses are evident in either state. Figure 3-7. Using a Microcontroller to Obtain HeartBeat Pulses in Either Output State PORT_M.x 1 Ro OUT SNSK 3 Microcontroller SNS 4 PORT_M.y SYNC 6 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. 3.9.3 Output Drive The OUT pin is active high and can sink or source up to 2 mA. When a large value of Cs (>20 nF) is used the OUT current should be limited to