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LM3915 Datasheet(PDF) 8 Page - Texas Instruments

Part No. LM3915
Description  LM3915 Dot/Bar Display Driver
Download  31 Pages
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI - Texas Instruments

LM3915 Datasheet(HTML) 8 Page - Texas Instruments

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LM3915
SNVS740C – JANUARY 2000 – REVISED MARCH 2013
www.ti.com
FUNCTIONAL DESCRIPTION
The simplified LM3915 block diagram is included 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 3 dB 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 at least 1.5V below V+ and no lower
than V.
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:
(1)
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. For correct operation, reference load current should
be between 80
μA and 5 mA. Load capacitance should be less than 0.05 μF.
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.
The LM3915 outputs are current-limited NPN transistors as shown below. An internal feedback loop regulates
the transistor drive. Output current is held at about 10 times the reference load current, independent of output
voltage and processing variables, as long as the transistor is not saturated.
8
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Copyright © 2000–2013, Texas Instruments Incorporated
Product Folder Links: LM3915


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