LM3915

SNVS740C – JANUARY 2000 – REVISED MARCH 2013

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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

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

(1)

Since the 120

μA current (max) from the adjust terminal represents an error term, the reference was designed to

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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