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Si1102-A-GM Arkusz danych(PDF) 8 Page - Silicon Laboratories |
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Si1102-A-GM Arkusz danych(HTML) 8 Page - Silicon Laboratories |
8 / 16 page Si1102 8 Rev. 1.0 3.2. Choice of LED and LED Current In order to maximize detection distance, the use of an infrared LED is recommended. However, red (visible) LEDs are viable in applications where a visible flashing LED may be useful and a shorter detection range is acceptable. White LEDs have slow response and do not match the Si1102’s spectral response well; they are, therefore, not recommended. To maximize proximity detection distance, an LED with a peak current handling of 400 mA is recommended. With careful system design, the duty cycle can be made low, enabling most LEDs to handle this peak current while keeping the LED's average current draw on the order of a few microamperes. Another consideration when choosing an LED is the LED's half-angle. An LED with a narrow half-angle focuses the available infrared light using a narrower beam. When the concentrated infrared light encounters an object, the reflection is much brighter. Detection of human-size objects one meter away can be achieved when choosing an LED with a narrower half-angle and coupling it with an infrared filter on the enclosure. 3.3. Power-Supply Transients Despite the Si1102's extreme sensitivity, it has good immunity from power-supply ripple, which should be kept below 50 mVpp for optimum performance. Power-supply transients (at the given amplitude, frequency, and phase) can cause either spurious detections or a reduction in sensitivity if they occur at any time within the 300 µs prior to the LED being turned on. Supply transients occurring after the LED has been turned off have no effect since the proximity state is latched until the next cycle. The Si1102 itself produces sharp current transients on its VDD pin, and, for this reason, must also have a low-impedance capacitor on its supply pins. Current transients at the Si1102 supply can be up to 20 mA. The typical LED current peak of 400 mA can induce supply transients well over 50 mVpp, but those transients are easy to decouple with a simple R-C filter because the duty-cycle-averaged LED current is quite low. The TXO output can be allowed to saturate without problem. Only the first 10 µs of the LED turn-on time are critical to the detection range; this further lessens the need for large reservoir capacitors on the LED supply. In most applications, 10 µF is adequate. If the LED is powered directly from a battery or limited-current source, it is desirable to minimize the load peak current by adding a resistor in series with the LED’s supply capacitor. If a regulated supply is available, the Si1102 should be connected to the regulator’s output and the LED to the unregulated voltage, provided it is less than 7 V. There is no power-sequencing requirement between VDD and the LED supply. 3.4. Mechanical and Optical Implementation It is important to have an optical barrier between the LED and the Si1102. The reflection from objects to be detected can be very weak since, for small objects within the LED's emission angle, the amplitude of the reflected signal decreases in proportion with the fourth power of the distance. The receiver can detect a signal with an irradiance of 1 µW/cm2. An efficient LED typically can drive to a radiant intensity of 100 mW/sr. Hypothetically, if this LED were to couple its light directly into the receiver, the receiver would be unable to detect any 1 µW/cm2 signal since the 100 mW/cm2 leakage would saturate the receiver. Therefore, to detect the 1 µW/cm2 signal, the internal optical coupling (e.g. internal reflection) from the LED to the receiver must be minimized to the same order of magnitude (decrease by 105) as the signal the receiver is attempting to detect. As it is also possible for some LEDs to drive a radiant intensity of 400 mW/sr, it is good practice to optically decouple the LED from the source by a factor of 106. If an existing enclosure is being reused and does not have dedicated openings for the LED and the Si1102, the proximity detector may still work if the optical loss factor through improvised windows (e.g. nearby microphone or fan holes) or semi-opaque material is not more than 90% in each direction. In addition, the internal reflection from an encased device's PMMA (acrylic glass) window (common in cellular telephones, PDAs, etc.) must be reduced through careful component placement. To reduce the optical coupling from the LED to the Si1102 receiver, the distance between the LED and the Si1102 should be maximized, and the distance between both components (LED and Si1102) to the PMMA window should be minimized. The detector can also work without a dedicated window if a semi-opaque plastic case is used. |
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