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CS209AYD14 데이터시트(PDF) 3 Page - Cherry Semiconductor Corporation |
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CS209AYD14 데이터시트(HTML) 3 Page - Cherry Semiconductor Corporation |
3 / 6 page 3 0.400 0.300 0.200 0.100 0.0 0.75 1.0 1.25 1.5 1.75 Distance To Object (in.) 2.5k W 5k W 7.5kW 12.5k W 15k W 17.5k W Object Detected Object Not Detected, L Unloaded. (T = 21 °C, VCC = 12V) Demodulator Voltage vs. Distance for Different RF Typical Performance Characteristics: continued The CS209A is a metal detector circuit which operates on the principle of detecting a reduction in Q of an inductor when it is brought into close proximity of metal. The CS209A contains an oscillator set up by an external parallel resonant tank and a feedback resistor connected between OSC and RF. (See Test and Applications Diagram) The impedance of a parallel resonant tank is highest when the frequency of the source driving it is equal to the tankÕs res- onant frequency. In the CS209A the internal oscillator operates close to the resonant frequency of the tank circuit selected. As a metal object is brought close to the inductor, the amplitude of the voltage across the tank gradually begins to drop. When the envelope of the oscillation reach- es a certain level, the IC causes the output stages to switch states. The detection is performed as follows: A capacitor con- nected to DEMOD is charged via an internal 30µA current source. This current, however, is diverted away from the capacitor in proportion to the negative bias generated by the tank at TANK. Charge is therefore removed from the capacitor tied to DEMOD on every negative half cycle of the resonant voltage. (See Figure 1) The voltage on the capacitor at DEMOD, a DC voltage with ripple, is then directly compared to an internal 1.44V reference. When the internal comparator trips it turns on a transistor which places a 23.6k½ resistor in parallel to the 4.8k½. The result- ing reference then becomes approximately 1.2V. This hys- teresis is necessary for preventing false triggering. The feedback potentiometer connected between OSC and RF is adjusted to achieve a certain detection distance range. The larger the resistance the greater the trip-point distance (See graph Demodulator Voltage vs Distance for Different RF). Note that this is a plot representative of one particular set-up since detection distance is dependent on the Q of the tank. Note also from the graph that the capaci- tor voltage corresponding to the greatest detection dis- tance has a higher residual voltage when the metal object Principle of Operation is well outside the trip point. Higher values of feedback resistance for the same inductor Q will therefore eventu- ally result in a latched-ON condition because the residual voltage will be higher than the comparatorÕs thresholds. As an example of how to set the detection range, place the metal object at the maximum distance from the inductor the circuit is required to detect, assuming of course the Q of the tank is high enough to allow the object to be within the ICÕs detection range. Then adjust the potentiometer to obtain a lower resistance while observing one of the CS209A outputs return to its normal state (see Test and Applications Diagram). Readjust the potentiometer slow- ly toward a higher resistance until the outputs have switched to their tripped condition. Remove the metal and confirm that the outputs switch back to their normal state. Typically the maximum distance range the circuit is capable of detecting is around 0.3 inch. The higher the Q, the higher the detection distance. For this application it is recommended to use a core which concentrates the magnetic field in only one direc- tion. This is accomplished very well with a pot core half. The next step is to select a core material with low loss fac- tor (inverse of Q). The loss factor can be represented by a resistance in series with the inductor which arises from core losses and is a function of frequency. The final step in obtaining a high Q inductor is the selec- tion of wire size. The higher the frequency the faster the decrease in current density towards the center of the wire. Thus most of the current flow is concentrated on the sur- face of the wire resulting in a high AC resistance. LITZ wire is recommended for this application. Considering the many factors involved, it is also recommended to operate at a resonant frequency between 200 and 700kHz. The formula commonly used to determine the Q for par- allel resonant circuits is: QP @ R 2¹fRL |
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