AT42QT1012

Features • Number of Keys: – One, toggle mode (touch-on/touch-off), plus programmable auto-off delay and external cancel – 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 external capacitor (Cs) Interface: – Digital output, active high or active low (hardware configurable) Moisture tolerance: – Good Power: – 1.8V ~ 5.5V; 32 µA at 1.8V Package: – 6-pin SOT23-6 (3 x 3 mm) RoHS compliant – 8-pin UDFN/USON (2 x 2 mm) RoHS compliant Signal processing: – Self-calibration, auto drift compensation, noise filtering • • • • • • • One-channel Toggle-mode Touch Sensor IC with Power Management Functions AT42QT1012 • • • • • • • 9543D–AT42–08/10 1. Pinout and Schematic 1.1 1.1.1 Pinout Configurations 6-pin SOT23-6 OUT VSS SNSK 1 6 TIME VDD SNS QT1012 2 3 5 4 1.1.2 8-pin UDFN/USON Pin 1 ID SNSK N/C N/C VSS 1 2 3 4 8 SNS VDD TIME OUT QT1012 7 6 5 2 AT42QT1012 9543D–AT42–08/10 AT42QT1012 1.2 Pin Descriptions Pin Listing 8-Pin 5 4 1 8 7 6 2 3 Name OUT Vss SNSK SNS Vdd TIME N/C N/C Type O (1) P I/O I/O P I – – Description Output state. To switched circuit and output polarity selection resistor (Rop) Ground Sense pin. To Cs capacitor and to sense electrode Sense pin. To Cs capacitor and multiplier configuration resistor (Rm). Rm must be fitted and connected to either VSS or VDD. See Section 3.11.4 on page 13 for details. Power Timeout configuration pin. Must be connected to either VSS, VDD, OUT or an RC network. See Section 3.11 on page 11 for details. Not connected Not connected Do not connect Do not connect Cs + key Cs If Unused, Connect To... Table 1-1. 6-Pin 1 2 3 4 5 6 – – 1. I/O briefly on power-up I O Input only Output only, push-pull I/O P Input and output Ground or power 1.3 1.3.1 Schematics 6-pin SOT23-6 Figure 1-1. Basic Circuit Configuration (active high output, toggle on/off, no auto switch off) VDD Cby RS CS 4 3 5 SENSE ELECTRODE Note: bypass capacitor to be tightly wired between VDD and VSS and kept close to pin 5. OUT 1 VDD SNSK SNS Rop Rm VSS 2 TIME 6 3 9543D–AT42–08/10 1.3.2 8-pin UDFN/USON Figure 1-2. Basic Circuit Configuration (active high output, toggle on/off, no auto switch off) VDD Cby RS CS 8 2 1 7 SENSE ELECTRODE Note: bypass capacitor to be tightly wired between VDD and VSS and kept close to pin 7. OUT 5 VDD SNSK SNS Rop Rm 3 N/C TIME 6 N/C VSS 4 Re Figure 1-1 on page 3 and Figure 1-2, check the following sections for component values: • Cs capacitor (Cs) – see Section 4.2 on page 20 • Sample resistor (Rs) – see Section 4.3 on page 20 • Voltage levels – see Section 4.4 on page 20 • Output polarity selection resistor (Rop) – see Section 3.9 on page 10 • Rm resistor – see Section 3.11.2 on page 11 • Bypass capacitor (Cby) – see page 21 4 AT42QT1012 9543D–AT42–08/10 AT42QT1012 2. Overview of the AT42QT1012 2.1 Introduction The AT42QT1012 (QT1012) is a single key device featuring a touch on/touch off (toggle) output with a programmable auto switch-off capability. The QT1012 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 low cost, noncritical components are required for operation. With its tiny low-cost packages, this device can suit almost any product needing a power switch or other toggle-mode controlled function, especially power control of small appliances and battery-operated products. A unique “green” feature of the QT1012 is the timeout function, which can turn off power after a time delay. Like all QTouch™ devices, the QT1012 features automatic self-calibration, drift compensation, and spread-spectrum burst modulation in order to provide for the most reliable touch sensing possible. 2.2 Basic Operation Figure 1-1 on page 3 and Figure 1-2 on page 4 show basic circuits for the 6-pin and 8-pin devices. The QT1012 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 QT1012. 2.3 Electrode Drive Figure 2-1 on page 6 shows the sense electrode connections (SNS, SNSK) for the QT1012. For optimum noise immunity, the electrode should only be connected to the SNSK pin. In all cases the sample capacitor Cs should be much larger than the load capacitance (Cx). Typical values for Cx are 5 – 20 pF while Cs is usually 2.2 – 50 nF. Note: Cx is not a physical discrete component on the PCB, it is the capacitance of the touch electrode and wiring. It is show in Figure 2-1 on page 6 to aid understanding of the equivalent circuit. Increasing amounts of Cx destroy gain, therefore it is important to limit the amount of load 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. To endure that the correct output mode is selected at power-up, the OUT trace should also be carefully routed. 5 9543D–AT42–08/10 A series resistor, Rs, should be placed in line with SNSK to the electrode to suppress electrostatic discharge (ESD) and electromagnetic compatibility (EMC) effects. Figure 2-1. Sense Connections SENSE ELECTRODE RS CS 4 3 VDD 5 Cby OUT 1 VDD SNSK SNS Cx TIME 6 VSS 2 2.4 2.4.1 Sensitivity Introduction The sensitivity on the QT1012 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. 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 a 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. 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 QT1012 may be too sensitive. In this case gain can be easily lowered further by decreasing Cs. 6 AT42QT1012 9543D–AT42–08/10 AT42QT1012 3. Operation Specifics 3.1 3.1.1 Acquisition Modes Introduction The OUT pin of the QT1012 can be configured to be active high or active low. • If active high then: – “on” is high – “off” is low • If active low then: – “on” is low – “off” is high 3.1.2 OUT Pin The QT1012 runs in Low Power (LP) mode. 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). • If the touch is confirmed, the OUT pin is toggled and the QT1012 returns to LP mode (see Figure 3-1). • If the touch is not valid then the chip returns to LP mode but the OUT pin remains unchanged (see Figure 3-2 on page 7). Figure 3-1. Low Power Mode: Touch Confirmed (Output in Off Condition) Key touch Fast detect integrator Sleep ~80 ms SNSK Sleep Sleep OUT Figure 3-2. Low Power Mode: Touch Denied (Output in Off Condition) Key touch Fast detect integrator Sleep Sleep ~80 ms SNSK Sleep Sleep OUT 7 9543D–AT42–08/10 3.2 Detect Threshold The device detects a touch when the signal has crossed a threshold level. The threshold level is fixed at 10 counts. 3.3 Detect Integrator It is desirable to suppress detections generated by electrical noise or from quick brushes with an object. To accomplish this, the QT1012 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 QT1012, the required count is four. The DI can also be viewed as a “consensus filter” that requires four successive detections to create an output. 3.4 Recalibration Timeout If an object or material obstructs the sense electrode the signal may rise enough to create a detection, preventing further operation. To stop this, the sensor includes a timer which monitors detections. If a detection exceeds the timer setting, the sensor performs a full recalibration. This does not toggle the output state but ensures that the QT1012 will detect a new touch correctly. The timer is set to activate this feature after ~60s. This will vary slightly with Cs. 3.5 Forced Sensor Recalibration The QT1012 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 QT1012’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 a 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 3-3 on page 9) 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 QT1012 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. 8 AT42QT1012 9543D–AT42–08/10 AT42QT1012 Figure 3-3. Drift Compensation S ignal H ysteresis Threshold R eference Output The QT1012'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. 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 QT1012's response time is highly dependent on the 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. 3.8 Spread Spectrum The QT1012 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. 9 9543D–AT42–08/10 3.9 Output Polarity Selection The output (OUT pin) of the QT1012 can be configured to have an active high or active low output by means of the output configuration resistor Rop. The resistor is connected between the output and either Vss or Vdd (see Figure 3-4 and Table 3-1). A typical value for Rop is 100 k. Figure 3-4. Output Polarity (6-pin SOT23) SENSE ELECTRODE VDD Cby 100 nF 5 RS 3 SNSK VDD Rop Vop CS 4 SNS OUT 1 Rm VSS 2 TIME 6 Table 3-1. Output Configuration Function (Output Polarity) Active high Active low Name (Vop) Vss Vdd Note that some devices such as Digital Transistors have an internal biasing network that will naturally pull the OUT pin to its inactive state. If these are being used then the resistor Rop is not required (see Figure 3-5). Figure 3-5. Output Connected to Digital Transistor (6-pin SOT23) SENSE ELECTRODE VDD Cby 100 nF 5 RS 3 SNSK VDD Load 1 CS 4 SNS OUT Rm VSS 2 TIME 6 10 AT42QT1012 9543D–AT42–08/10 AT42QT1012 3.10 Output Drive The OUT pin 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