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LM3914N-1 Folha de dados(PDF) 7 Page - National Semiconductor (TI) |
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LM3914N-1 Folha de dados(HTML) 7 Page - National Semiconductor (TI) |
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7 / 19 page  Functional Description The simplifed LM3914 block diagram is to give the general idea of the circuit’s operation. A high input impedance buffer operates with signals from ground to 12V, and is protected against reverse and overvoltage signals. The signal is then applied to a series of 10 comparators; each of which is bi- ased to a different comparison level by the resistor string. In the example illustrated, the resistor string is connected to the internal 1.25V reference voltage. In this case, for each 125 mV that the input signal increases, a comparator will switch on another indicating LED. This resistor divider can be connected between any 2 voltages, providing that they are 1.5V below V + and no less than V−. If an expanded scale meter display is desired, the total divider voltage can be as little as 200 mV. Expanded-scale meter displays are more accurate and the segments light uniformly only if bar mode is used. At 50 mV or more per step, dot mode is usable. INTERNAL VOLTAGE REFERENCE The reference is designed to be adjustable and develops a nominal 1.25V between the REF OUT (pin 7) and REF ADJ (pin 8) terminals. The reference voltage is impressed across program resistor R1 and, since the voltage is constant, a constant current I 1 then flows through the output set resistor R2 giving an output voltage of: Since the 120 µA current (max) from the adjust terminal rep- resents an error term, the reference was designed to mini- mize changes of this current with V + and load changes. CURRENT PROGRAMMING A feature not completely illustrated by the block diagram is the LED brightness control. The current drawn out of the ref- erence voltage pin (pin 7) determines LED current. Approxi- mately 10 times this current will be drawn through each lighted LED, and this current will be relatively constant de- spite supply voltage and temperature changes. Current drawn by the internal 10-resistor divider, as well as by the ex- ternal current and voltage-setting divider should be included in calculating LED drive current. The ability to modulate LED brightness with time, or in proportion to input voltage and other signals can lead to a number of novel displays or ways of indicating input overvoltages, alarms, etc. MODE PIN USE Pin 9, the Mode Select input controls chaining of multiple LM3914s, and controls bar or dot mode operation. The fol- lowing tabulation shows the basic ways of using this input. Other more complex uses will be illustrated in the applica- tions. Bar Graph Display: Wire Mode Select (pin 9) directly to pin 3(V + pin). Dot Display, Single LM3914 Driver: Leave the Mode Select pin open circuit. Dot Display, 20 or More LEDs: Connect pin 9 of the first driver in the series (i.e., the one with the lowest input voltage comparison points) to pin 1 of the next higher LM3914 driver. Continue connecting pin 9 of lower input drivers to pin 1 of higher input drivers for 30, 40, or more LED displays. The last LM3914 driver in the chain will have pin 9 wired to pin 11. All previous drivers should have a 20k resistor in parallel with LED No. 9 (pin 11 to V LED). Mode Pin Functional Description This pin actually performs two functions. Refer to the simpli- fied block diagram below. DOT OR BAR MODE SELECTION The voltage at pin 9 is sensed by comparator C1, nominally referenced to (V + − 100 mV). The chip is in bar mode when pin 9 is above this level; otherwise it’s in dot mode. The com- parator is designed so that pin 9 can be left open circuit for dot mode. Taking into account comparator gain and variation in the 100 mV reference level, pin 9 should be no more than 20 mV below V + for bar mode and more than 200 mV below V+ (or open circuit) for dot mode. In most applications, pin 9 is ei- ther open (dot mode) or tied to V + (bar mode). In bar mode, pin 9 should be connected directly to pin 3. Large currents drawn from the power supply (LED current, for example) should not share this path so that large IR drops are avoided. DOT MODE CARRY In order for the display to make sense when multiple LM3914s are cascaded in dot mode, special circuitry has been included to shut off LED No. 10 of the first device when DS007970-4 Block Diagram of Mode Pin Description DS007970-5 *High for bar www.national.com 7 |
Nº de peça semelhante - LM3914N-1 |
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Descrição semelhante - LM3914N-1 |
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