LM3915 datasheet(8/25 Pages) NSC

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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 imped-

ance buffer operates with signals from ground to 12V, and is

protected against reverse and overvoltage signals. The sig-

nal 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 con-

nected 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 I

1 then flows through the output set resistor

R2 giving an output voltage of:

00510405

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 be-

tween 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. Ap-

proximately 10 times this current will be drawn through each

lighted LED, and this current will be relatively constant de-

spite 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 in-

cluded in calculating LED drive current. The ability to modu-

late LED brightness with time, or in proportion to input volt-

age 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 tran-

sistor 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 satu-

rated.

LM3915 Output Circuit

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Outputs may be run in saturation with no adverse effects,

making it possible to directly drive logic. The effective satu-

ration resistance of the output transistors, equal to R

E plus

the transistors’ collector resistance, is about 50

Ω. It’s also

possible to drive LEDs from rectified AC with no filtering. To

avoid oscillations, the LED supply should be bypassed with a

2.2 µF tantalum or 10 µF aluminum electrolytic capacitor.

MODE PIN USE

Pin 9, the Mode Select input, permits chaining of multiple

LM3915s, 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 applica-

tions.

Bar Graph Display: Wire Mode Select (pin 9) directly to pin

3(V

+ pin).

Dot Display, Single LM3915 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 LM3915 driver.

Continue connecting pin 9 of lower input drivers to pin 1 of

higher input drivers for 30 or more LED displays. The last

LM3915 driver in the chain will have pin 9 left open. All

previous drivers should have a 20k resistor in parallel with

LED #9 (pin 11 to V

LED).

Mode Pin Functional Description

This pin actually performs two functions. Refer to the simpli-

fied block diagram below.

Block Diagram of Mode Pin Function

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*High for bar

LM3915 Datasheet(PDF) 8 Page - National Semiconductor (TI)

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