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PCF88831 Datasheet(PDF) 6 Page - NXP Semiconductors |
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PCF88831 Datasheet(HTML) 6 Page - NXP Semiconductors |
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6 / 25 page  PCF8883 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved. Product data sheet Rev. 2 — 8 March 2011 6 of 25 NXP Semiconductors PCF8883 Capacitive proximity switch with auto-calibration If the capacitance on pin IN increases, the discharge time tdch increases too. Therefore it will take longer for the voltage on the corresponding comparator to drop below Vref. Only once this happens, the comparator output will become LOW and this results in a pulse on CDN discharging the external capacitor CCPC slightly. Thus most pulses will now be given by CUP. Without further action, capacitor CCPC would then fully charge. However, a chip-internal automatic calibration mechanism that is based on a voltage controlled sink current (Isink) connected to pin IN attempts to equalize the discharge time tdch with the internal reference discharge time tdch(ref). The current source is controlled by the voltage on CCPC which causes the capacitance on pin IN to be discharged more quickly in the case that the voltage on CCPC is rising, thereby compensating for the increase in capacitance on input pin IN. This arrangement constitutes a closed-loop control system that constantly attempts to equalize the discharge time tdch with tdch(ref). This allows compensating for slow changes in capacitance on input pin IN. Fast changes due to an approaching hand for example will not be compensated. In the equilibrium state the discharge times are equal and the pulses alternate between CUP and CDN. From this also follows that an increase in capacitor value CCPC results in a smaller voltage change per pulse CUP or CDN. Thus the compensation due to internal current sink source Isink is slower and therefore the sensitivity of the sensor will increase. Likewise a decrease in capacitor CCPC will result in a lower sensitivity. (For further information see Section 13.) CSENS = sensing plate capacitance. CCABLE = cable capacitance. RC = external discharge (pull-down) resistor. RF = low pass filter resistor. CF = low pass filter capacitor. Fig 4. Functional principle of the PCF8883 COUNTER LOGIC OSCILLATOR VDD VDD(INTREGD) OUT TYPE CLIN CPC VSS IN SENSOR LOGIC CF SENSING PLATE COAXIAL CABLE CSENS RF RC CCABLE PCF8883 VOLTAGE REGULATOR 013aaa075 fs |
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