CS209A Datasheet, PDF (4/8 Pages) Cherry Semiconductor Corporation – Proximity Detector

CS209A
TYPICAL PERFORMANCE CHARACTERISTICS
8
(T = 25°C, V_CC = 12 V)
6
6.5
(V_CC = 12 V, R_LOAD = 1.0 kΩ)
5.5
4
0.1 kΩ
2
0
4
8
12
16
20
Output Load (kΩ)
Figure 2. Output Switching Delay vs.
Output Load
4.5
3.5
2.5
−40 −20 0
20 40 60 80 100 120
Temperature (°C)
Figure 3. Output Switching Delay vs.
Temperature
Object
Detected
1.75
(T = 25°C, V_CC = 12 V)
1.5
1.25
1.0
2.5 kΩ 5.0 kΩ 7.5 kΩ
12.5 kΩ
15 kΩ
17.5 kΩ
Object Not
Detected,
L Unloaded
0.75
0
0.100
0.200
0.300
Distance To Object (in.)
Figure 4. Demodulator Voltage vs. Distance for Different RF
0.400
PRINCIPLE OF OPERATION
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 Figure 5.) The impedance of a parallel
resonant tank is highest when the frequency of the source
driving it is equal to the tank’s resonant 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 reaches a certain level, the IC causes the
output stages to switch states.
The detection is performed as follows: A capacitor
connected to DEMOD is charged via an internal 30 μA 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 6) The voltage on the capacitor at
DEMOD, a DC voltage with ripple, is then directly compared
to an internal 1.44 V reference. When the internal comparator
trips it turns on a transistor which places a 23.6 kΩ resistor in
parallel to the 4.8 kΩ. The resulting reference then becomes
approximately 1.2 V. This hysteresis is necessary for preventing
false triggering.
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