Datasheet

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 I

then flows through the output set resistor R2 giving an output voltage of:

Since the 120μA 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

LED

Product Folder Links: LM3914