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LM3914N-1 Datasheet(PDF) 9 Page - Texas Instruments

Part # LM3914N-1
Description  LM3914 Dot/Bar Display Driver
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
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LM3914N-1 Datasheet(HTML) 9 Page - Texas Instruments

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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.
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.
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.
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
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
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