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LM3914N Folha de dados(PDF) 14 Page - National Semiconductor (TI) |
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LM3914N Folha de dados(HTML) 14 Page - National Semiconductor (TI) |
14 / 19 page Typical Applications (Continued) Application Hints Three of the most commonly needed precautions for using the LM3914 are shown in the first typical application drawing showing a 0V–5V bar graph meter. The most difficult prob- lem occurs when large LED currents are being drawn, espe- cially in bar graph mode. These currents flowing out of the ground pin cause voltage drops in external wiring, and thus errors and oscillations. Bringing the return wires from signal sources, reference ground and bottom of the resistor string (as illustrated) to a single point very near pin 2 is the best so- lution. Long wires from V LED to LED anode common can cause os- cillations. Depending on the severity of the problem 0.05 µF to 2.2 µF decoupling capacitors from LED anode common to pin 2 will damp the circuit. If LED anode line wiring is inac- cessible, often similar decoupling from pin 1 to pin 2 will be sufficient. If LED turn ON seems slow (bar mode) or several LEDs light (dot mode), oscillation or excessive noise is usually the prob- lem. In cases where proper wiring and bypassing fail to stop oscillations, V + voltage at pin 3 is usually below suggested limits. Expanded scale meter applications may have one or both ends of the internal voltage divider terminated at rela- tively high value resistors. These high-impedance ends should be bypassed to pin 2 with at least a 0.001 µF capaci- tor, or up to 0.1 µF in noisy environments. Power dissipation, especially in bar mode should be given consideration. For example, with a 5V supply and all LEDs programmed to 20 mA the driver will dissipate over 600 mW. In this case a 7.5 Ω resistor in series with the LED supply will cut device heating in half. The negative end of the resistor should be bypassed with a 2.2 µF solid tantalum capacitor to pin 2 of the LM3914. Turning OFF of most of the internal current sources is ac- complished by pulling positive on the reference with a cur- rent source or resistance supplying 100 µA or so. Alternately, the input signal can be gated OFF with a transistor switch. Other special features and applications characteristics will be illustrated in the following applications schematics. Notes have been added in many cases, attempting to cover any special procedures or unusual characteristics of these appli- cations. A special section called “Application Tips for the LM3914 Adjustable Reference” has been included with these schematics. 20-Segment Meter with Mode Switch DS007970-14 *The exact wiring arrangement of this schematic shows the need for Mode Select (pin 9) to sense the V+ voltage exactly as it appears on pin 3. Programs LEDs to 10 mA www.national.com 14 |
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Descrição semelhante - LM3914N |
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