LM3916 datasheet(7/27 Pages) NSC

7 / 27 page

Functional Description

The simplified LM3916 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. As the input voltage

varies from 0 to 1.25, the comparator outputs are driven low

one by one, switching on the LED indicators. The resistor di-

vider 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 I

1 then flows through the output set resistor

R2 giving an output voltage of:

Since the 120 µA current (max) from the adjust terminal rep-

resents an error term, the reference was designed to mini-

mize 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 ref-

erence voltage pin (pin 7) determines LED current. Approxi-

mately 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 ex-

ternal 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 LM3916 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 ref-

erence load current, independent of output voltage and pro-

cessing variables, as long as the transistor is not saturated.

Outputs may be run in saturation with no adverse effects,

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

tion resistance of the output transistors, equal to R

E plus the

transistors’ collector resistance, is about 50

Ω. It’s also pos-

sible 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 de-

vices, 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 LM3916 Driver: Leave the Mode Select

pin open circuit.

Dot Display, 20 or More LEDs: Connect pin 9 of the

first

drivers in the series (i.e., the one with the lowest input volt-

age comparison points) to pin 1 of the next higher LM3916

driver. Continue connecting pin 9 of lower input drivers to pin

1 of higher input drivers for 30 or more LED displays. The

last LM3916 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.

DS007971-5

LM3916 Output Circuit

DS007971-6

Block Diagram of Mode Pin Function

DS007971-7

*High for bar

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