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LM3914N-1 Datasheet(PDF) 9 Page - Texas Instruments |
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LM3914N-1 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 29 page ![]() LM3914 www.ti.com SNVS761B – JANUARY 2000 – REVISED MARCH 2013 FUNCTIONAL DESCRIPTION The simplified 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 biased 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 125mV 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 200mV. Expanded-scale meter displays are more accurate and the segments light uniformly only if bar mode is used. At 50mV 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 I1 then flows through the output set resistor R2 giving an output voltage of: Since the 120 μA current (max) from the adjust terminal represents an error term, the reference was designed to minimize 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 reference voltage pin (pin 7) determines LED current. Approximately 10 times this current will be drawn through each lighted LED, and this current will be relatively constant despite supply voltage and temperature changes. Current drawn by the internal 10-resistor divider, as well as by the external 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 following tabulation shows the basic ways of using this input. Other more complex uses will be illustrated in the applications. 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 VLED). Copyright © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM3914 |
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