Suppliers put a great deal of effort. The analog view: examining TI s audio capacitivetouch

Transcription

1 The analog view: examining TI s audio capacitivetouch boosterpack demo board A look at the analog architecture provides insight into the design choices Texas Instruments made for its audio capacitive-touch booster-pack demo board. By Steve Taranovich Senior Technical Editor image: THINKSTOCK Suppliers put a great deal of effort into making their demo boards show off the best performance possible for their products. They thoroughly test layouts, and designers implement many iterations to squeeze out the last microvolt of noise, the extra fraction of a decibel of dynamic range, or some other important performance parameter of the ICs or functions the manufacturer wants to highlight. Texas Instruments audio capacitive-touch booster-pack demo board employs a DSP and a microcontroller (Reference ). The microcontroller provides analog assistance to simplify and enhance design, and the DSP provides higher-level assistance to what essentially becomes an MP3 player. Designers need first to understand the capacitive-touch mechanism (Figure ). A finger disrupts the electric field, which sees the finger as a conductive foreign object, thus changing the capacitance. A ground plane beneath the touchpad helps shield the structure from other electronics in the design, such as microcontrollers, DSPs, and clocks. The MSP30 microcontroller constantly monitors and tracks slowly changing environmental effects, such as temperature and humidity, which can affect operation. [ ] June, 0 EDN 7

2 To measure the sensor s capacitance, the MSP30 counts the number of relaxation-oscillator cycles within a fixed gate period (Figure ), comparing the fixed-timing domain of the reference and the variable-timing domain of the finger s capacitance. You can implement the relaxation oscillator with a comparator or, in the case of the MSP30, using a pin-oscillator feature, in which the frequency of the oscillation is a function of the resistance and the capacitance of the circuit. The capacitance is the intended variable and increases with a touch. In the time domain, the rise and fall times increase. Conversely, in the frequency domain, the frequency decreases. With an increase in capacitance, the number of relaxation-oscillator cycles decreases within the fixed gate time. SENSOR GROUND AT A GLANCE TI s audio capacitive-touch booster-pack demo board employs a DSP and a microcontroller. The high integration in the microcontroller allows the luxury of a relatively straightforward capacitive sensor input. The microcontroller provides analog assistance to simplify and enhance the design; the DSP provides the higher-level assistance to what essentially becomes an MP3 player. The relaxation oscillator feeds into a Schmitt-trigger circuit that generates a square packet and then counts the pulses. Texas Instruments offers a free Figure An opened capacitor structure acts as a single-touch sensitive capacitor sensor in capacitive-touch systems. capacitive-touch library, which enables buttons, sliders, wheels, and proximity sensors on all MSP30 microcontrollers, eliminating the need for developers to create complex touch-sensing algorithms (Reference ). Figure 3 shows a bus-connection diagram of all elements in the audio capacitive-touch booster pack. Both USB ports provide V, but the booster pack requires to source the.3,.8, and power rails; the 3V for OLED backlight; and the for the MSP30. The ACTBP USB connector feeds a to dc/dc converter to provide from the V USB port. The device uses the TPS660 synchronous-step-down converter, which touts efficiency greater than 90% with a -μa quiescent current. It provides as much as 600-mA output current from one lithium-ion cell. The device s high switching frequency allows the use of small inductors and capacitors to achieve a small product that comes in a TSOT-3- or a -mm TXFN package (Figure a). The MSP30 launchpad connector provides power to the launchpad and the booster pack. The launchpad provides for the main powersupply voltage from the V USB port. The A, TPS79633 low-dropout, low-power, fixed linear regulator has a power-supply rejection ratio of 9 db at 00 khz and noise of 0 μv rms. It comes in an SOT-3-6 package. The TPS79633 powers most analog ICs, TIMEBASE RELAXATION OSCILLATOR REFERENCE COUNTER FREQUENCY OF C C <C FREQUENCY OF C TIMEBASE SYSTEM CLOCK INTERVAL GATE TIME Figure This design uses relaxation oscillation to sense capacitance. C represents the large capacitance of a finger close to the sensor. 30 EDN June, 0 [ ]

3 MSP30 UART AND GPIO GPIOs GPIOs Cx DSP LEDs CAPACITANCE- TOUCH ELEMENTS Figure 3 The audio capacitive-touch booster pack bus-connection block diagram shows a connection to an MSP30 microcontroller launchpad board. All pertinent bus structures in the design are shown. such as the AIC30 codec, and the TS3AE audio switch (Figure b and Reference 3). Other available power supplies for the MSP30 include the TPS790 (Figure c and Reference ) and the TPS7780 (Figure d). The TPS mA low-dropout, low-power linear voltage regulator features a power-supply rejection ratio of 0 db at 0 khz in a small-outline SOT3-6 package. This device is stable with a small -µf ceramic capacitor on the output. The low noise and fast response time make the TPS790 a good choice for the.8v supply. The TPS7780 low-dropout regulator, in an 8-pin SOIC package, is designed to have a fast transient response and be stable with a 0-µF, lowequivalent-series-resistance capacitor. The device converts the 3.3 to.3v supply for the DSP (running at up to 00 MHz) digital core voltage and other core supplies in the DSP. The quiescent current is independent of output loading, making the TPS7780 a good choice for a low-power system. USB CONTROLLER I S AND I C I C MMCSD CONTROLLER USB AIC30 AUDIO CODEC OLED SCREEN SD CARD layout was also used on some other reference designs, so the hardware reuse saved time and allowed for the board to be introduced to designers as quickly as possible. This flexible, low-power, low-voltage stereo audio codec has programmable inputs and outputs, a lowpower analog-bypass mode, power-tune features, fixed predefined and programmable microphone bias, parameterizable signal-processing blocks, an integrated programmable phase-locked loop, integrated low-dropout regulators, and flexible digital interfaces. Its high level of integration also makes this codec a space saver (Figure ). The TS3AE autonomous audio switch with headset-type detection detects the presence of an analog microphone and switches a system s analogmicrophone pin between connectors in an audio stereo jack (Figure 6). Some connector manufacturers allow you to swap the microphone connection in a stereo connector with the ground connection. When the IC detects a certain configuration, the device automatically connects the microphone line to the appropriate pin. The device also reports the presence of an analog microphone on an audio stereo jack and takes up little space on the PCB. The demo board also includes four analog I C devices, which the C3 DSP controls an audio codec, an OLED (organic light-emitting-diode) display, the power supply s core power monitor, and the headset detector and Audio choices The booster pack has a headset jack for stereo-headset output and headsetmicrophone input. To support the various types of headsets, the TS3AE autonomous audio switch with headsettype detection connects the headset jack and the stereo audio codec. The designers of the demo board chose this codec because of its low power and because it allowed them to reuse software from other demo boards. Software reuse is a great time saver. The [ ] June, [ 0 EDN 3]

4 USB IC 6 TPS660DDCT VIN SW L 9. μh 3.9V D MBR00 J 3 R C 73.7 μf R C 9 7 GND 8.k C 7 C 7 00 pf 3 EN FB (a) V REF 0.6V R 0 3.3k R TO IC 9 TPS79633DCQ C 87 C 88 EN GND FB μf P.7 POWER C 83 C ENABLE μf μf R 8 R 9 V DD C 8.7 μf R.7k R 6.7k P.7 POWER ENABLE (b) C 8 μf 3.3 TO.8V C 80 IC 8 TPS790DCQ EN GND FB R 0 k C pf.8v.8v C 8 μf (c) V REF.6V R 30k (d) C 76.7 μf 3 IC TPS7780 EN FB/NC GND PG V REF.83V 3.3 TO.3V R 3 0k TARGET POWER GOOD R 0.7k C 77.7 μf R 0k.3V C 78.7 μf.3v Figure The TPS660 provides USB input to (a). The TPS79633 powers most analog ICs, such as the AIC30 codec, and the TS3AE audio switch (b). The TPS790 provides 3.3 to.8v (c). The TPS7780 provides 3.3 to.3v (d). 3 EDN June, 0 [ ]

5 .8V.8V R R J C C C C J R 3 C 6 C 8 C 9 C 7 R MICP MICN HP LEFT HP RIGHT R.k R 9 7k R k C μf C 0.7 μf C 3 00 μf C 00 μf R 0 7k C 3 C 06 C 6 9 MICBIAS 3 IN_L IN_R IN_L 6 IN_R 0 IN3_L IN3_R HPL 7 HPR LOL 3 LOR 30 3 LDO_SEL GPIO 8 REF C 7 IOVSS DVSS IOVDD DVDD AVDD LDO_IN IC TLV30AIC30IRHBT TARGET POWER GOOD RESET BCLK IS CLK 3 IS FS WCLK IS DX DIN IS RX DOUT MCLK MCLK SCL/SSZ 9 IC SCL 0 IC SDA SDA/MOSI SPI_SEL AIC MODE SEL MISO 8 MIC DET SCLK/MIC_DET AVSS P R 6.7k R.7k (a) R TP L BLMAGSND L MCLK C R VCC OFFA IC DSX3G MHz 3 CLK GND BLMAGSND R USB CLK MHz (b) (d) R CPU CLK MHz J 3 P.0 J J P. P. P. P. P.6 P.7 J 6 P.3 R R 7 R 8 R78 LED D 8 LED 6 D 9 LED 7 D 7 LED 8 D 6 J 7 P. D D 0 D D 3 J 8 P. P.3 LED LED LED 3 LED R 8 R 8 R 86 R 83 (c) P.0 R 87 D LED 9 Figure To support the various types of headsets, the TS3AE autonomous audio switch with headset-type detection connects the headset jack and the stereo audio codec (a), the BLMAGSND inductors (b), the capacitive-touch sensor (c), and the - MHz DSX3G clock (d). 3 EDN June, 0 [ ]

6 MICROPHONE BIAS MICROPHONE MICROPHONE REFERENCE C C 3 C R MIC N /HP REFERENCE V DD SCL SDA I C ADDRESS SELECT MICROPHONE PRESENT DETECTION TRIGGER MIC P S CONTROL LOGIC TS3AE Q Q SLEEVE SENSE RING SENSE SLEEVE RING TIP SENSE TIP RING Figure 6 The TS3AE autonomous audio switch with headset-type detection detects the presence of an analog microphone and switches a system s analog-microphone pin between connectors in an audio stereo jack. switch. The MSP30 sends UART commands to the DSP to configure the I C devices writing characters to the OLED display, for example. The INA9 provides the DSP s core power usage to maintain efficient power management. Using the INA9, you can characterize the CMOS DSP s power consumption on the application board when application code is running. You can then tailor the application code, code structure, and operating frequency for minimum power consumption. The zero-drift, bidirectional INA9 current/power monitor reports current, voltage, and power through the I C bus and comes in an SOT-3-8 package.edn References Maxfield, Clive, TI announces audio capacitive touch BoosterPack for ultralow-power MSP30 MCUs, EE Times, March 7, 0, Capacitive Touch Library, Programmer s Guide SLAA90, Texas Instruments, April 0, 3 TPS79633 Ultralow-Noise, High- PSRR, Fast, RF, A Low-Dropout Linear Regulators, Texas Instruments, September 00, TPS790 Ultralow-Noise, High- PSRR, Fast, RF, 00-mA Low-Dropout Linear Regulators, Texas Instruments, August 00, You can reach Senior Technical Editor Steve Taranovich at and steve.taranovich@ ubm.com. [ ] June, 0 EDN 3