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TL/H/7971

LM3916 Dot/Bar Display Driver

February 1995

LM3916 Dot/Bar Display Driver

General Description

The LM3916 is a monolithic integrated circuit that senses

analog voltage levels and drives ten LEDs, LCDs or vacuum

fluorescent displays, providing an electronic version of the

popular VU meter. One pin changes the display from a bar

graph to a moving dot display. LED current drive is regulat-

ed and programmable, eliminating the need for current limit-

ing resistors. The whole display system can operate from a

single supply as low as 3V or as high as 25V.

The IC contains an adjustable voltage reference and an ac-

curate ten-step voltage divider. The high-impedance input

buffer accepts signals down to ground and up to within 1.5V

of the positive supply. Further, it needs no protection

against inputs of g35V. The input buffer drives 10 individual

comparators referenced to the precision divider. Accuracy is

typically better than 0.2 dB.

Audio applications include average or peak level indicators,

and power meters. Replacing conventional meters with an

LED bar graph results in a faster responding, more rugged

display with high visibility that retains the ease of interpreta-

tion of an analog display.

The LM3916 is extremely easy to apply. A 1.2V full-scale

meter requires only one resistor in addition to the ten LEDs.

One more resistor programs the full-scale anywhere from

1.2V to 12V independent of supply voltage. LED brightness

is easily controlled with a single pot.

The LM3916 is very versatile. The outputs can drive LCDs,

vacuum fluorescents and incandescent bulbs as well as

LEDs of any color. Multiple devices can be cascaded for a

dot or bar mode display for increased range and/or resolu-

tion. Useful in other applications are the linear LM3914 and

the logarithmic LM3915.

Features

YFast responding electonic VU meter

YDrivers LEDs, LCDs, or vacuum fluorescents

YBar or dot display mode externally selectable by user

YExpandable to displays of 70 dB

YInternal voltage reference from 1.2V to 12V

YOperates with single supply of 3V to 25V

YInputs operate down to ground

YOutput current programmable from 1 mA to 30 mA

YInput withstands g35V without damage or false outputs

YOutputs are current regulated, open collectors

YDirectly drives TTL or CMOS

YThe internal 10-step divider is floating and can be refer-

enced to a wide range of voltages

The LM3916 is rated for operation from 0§Ctoa

70§C. The

LM3916N is available in an 18-lead molded DIP package.

Typical Applications

0V TO 10V VU Meter

TL/H/7971– 1

VREF e1.25V #1aR2

R1 JaR2 c80 mA

ILED e12.5V

R1 aVREF

2.2 kX

Note 1: Capacitor C1 is required if

leads to the LED supply are 6×or

longer.

Note 2: Circuit as shown is wired for

dot mode. For bar mode, connect pin

9 to pin 3. VLED must be kept below

7V or dropping resistor should be

used to limit IC power dissipation.

C1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.

Absolute Maximum Ratings

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales

Office/Distributors for availability and specifications.

Power Dissipation (Note 5)

Molded DIP (N) 1365 mW

Supply Voltage 25V

Voltage on Output Drivers 25V

Input Signal Overvoltage (Note 3) g35V

Divider Voltage b100 mV to Va

Reference Load Current 10 mA

Storage Temperature Range b55§Ctoa

150§C

Lead Temperature (Soldering, 10 seconds) 260§C

Electrical Characteristics (Notes 1 and 3)

Parameter Conditions (Note 1) Min Typ Max Units

COMPARATORS

Offset Voltage, Buffer and First 0V sVRLO eVRHI s12V, 3 10 mV

Comparator ILED e1mA

Offset Voltage, Buffer and Any Other Comparator 0V sVRLO eVRHI s12V, ILED e1 mA 3 15 mV

Gain (DILED/DVIN)I

(REF) e2 mA, ILED e10 mA 3 8 mA/mV

Input Bias Current (at Pin 5) 0V sVIN s(Vab1.5V) 25 100 nA

Input Signal Overvoltage No Change in Display b35 35 V

VOLTAGE DIVIDER

Divider Resistance Total, Pin 6 to 4 8 12 17 kX

Relative Accuracy (Input Change (Note 2)

Between Any Two Threshold Points) b1dBsV

IN s3 dB 0.75 1.0 1.25 dB

b7dBsV

IN sb1 dB 1.5 2.0 2.5 dB

b10 dB sVIN sb7 dB 2.5 3.0 2.5 dB

Absolute Accuracy (Note 2)

VIN e2, 1, 0, b1dB b

0.25 a0.25 dB

VIN eb

3, b5dB b

0.5 a0.5 dB

VIN eb

7, b10, b20 dB b1a1dB

VOLTAGE REFERENCE

Output Voltage 0.1 mAsIL(REF) s4mA, 1.2 1.28 1.34 V

VaeVLED e5V

Line Regulation 3V sVas18V 0.01 0.03 %/V

Load Regulation 0.1 mA sIL(REF) s4mA, 0.4 2 %

VaeVLED e5V

Output Voltage Change with Temperature 0§CsTAsa70§C,IL(REF) e1 mA, 1 %

VaeVLED e5V

Adjust Pin Current 75 120 mA

OUTPUT DRIVERS

LED Current VaeVLED e5V, IL(REF) e1 mA 7 10 13 mA

LED Current Difference (Between Largest VLED e5V, ILED e2 mA 0.12 0.4 mA

and Smallest LED Currents) VLED e5V, ILED e20 mA 1.2 3 mA

LED Current Regulation 2V sVLED s17V ILED e2 mA 0.1 0.25 mA

ILED e20 mA 1 3 mA

Dropout Voltage ILED(ON) e20 mA @VLED e5V, 1.5 V

DILED e2mA

Saturation Voltage ILED e2.0 mA, IL(REF) e0.4 mA 0.15 0.4 V

Output Leakage, Each Collector Bar Mode (Note 4) 0.1 100 mA

Output Leakage Dot Mode (Note 4)

Pins 10–18 0.1 100 mA

Pin 1 60 150 450 mA

Electrical Characteristics (Note 1) (Continued)

Parameter Conditions (Note 1) Min Typ Max Units

SUPPLY CURRENT

Standby Supply Current Vaea

5V, IL(REF) e0.2 mA 2.4 4.2 mA

(All Outputs Off) Vaea

20V, IL(REF) e1.0 mA 6.1 9.2 mA

Note 1: Unless otherwise stated, all specifications apply with the following conditions:

DC sVas20 VDC b0.015V sVRLO s12 VDC TAe25§C, IL(REF) e0.2 mA, pin 9 connected to pin 3 (bar mode).

DC sVLED sVaVREF,V

RHI,V

RLO s(Vab1.5V) For higher power dissipations, pulse testing is used.

b0.015V sVRHI s12 VDC 0V sVIN sVab1.5V

Note 2: Accuracy is measured referred to a3dBea

10.000 VDC at pin 5, with a10.000 VDC at pin 6, and 0.000 VDC at pin 4. At lower full-scale voltages, buffer

and comparator offset voltage may add significant error. See table for threshold voltages.

Note 3: Pin 5 input current must be limited to g3 mA. The addition of a 39k resistor in series with pin 5 allows g100V signals without damage.

Note 4: Bar mode results when pin 9 is within 20 mV of Va. Dot mode results when pin 9 is pulled at least 200 mV below Va. LED Ý10 (pin 10 output current) is

disabled if pin 9 is pulled 0.9V or more below VLED.

Note 5: The maximum junction temperature of the LM3916 is 100§C. Devices must be derated for operation at elevated temperatures. Junction to ambient thermal

resistance is 55§C/W for the molded DIP (N package).

LM3916 Threshold Voltage (Note 2)

dB Volts dB Volts

Min Typ Max Min Typ Max

3 9.985 10.000 10.015 b3g(/2 4.732 5.012 5.309

2g(/4 8.660 8.913 9.173 b5g(/2 3.548 3.981 4.467

1g(/4 7.718 7.943 8.175 b7g1 2.818 3.162 3.548

0g(/4 6.879 7.079 7.286 b10 g1 1.995 2.239 2.512

b1g(/2 5.957 6.310 6.683 b20 g1 0.631 0.708 0.794

Typical Performance Characteristics

Temperature

Supply Current vs

Current vs Temperature

Operating Input Bias

Temperature

Reference Voltage vs

Current vs Temperature

Reference Adjust Pin

Dropout

LED Current-Regulation

Voltage

LED Driver Saturation

Signal Range (Pin 5)

Input Current Beyond

Referenced Loading

LED Current vs

Regulation

LED Driver Current

TL/H/7971– 2

vs Temperature

Total Divider Resistance

Common-Mode Limits Output Characteristics

TL/H/7971– 3

Block Diagram (Showing Simplest Application)

TL/H/7971– 4

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 resis-

tor 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

divider can be connected between any 2 voltages, providing

that they are at least 1.5V below Vaand no lower than Vb.

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 I1then flows through the output set resis-

tor R2 giving an output voltage of:

VOUT eVREF #1aR2

R1 JaIADJ R2

TL/H/7971– 5

Since the 120 mA current (max) from the adjust terminal

represents an error term, the reference was designed to

minimize changes of this current with Vaand load changes.

For correct operation, reference load current should be be-

tween 80 mA and 5 mA. Load capacitance should be less

than 0.05 mF.

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 dis-

plays 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

reference load current, independent of output voltage and

processing variables, as long as the transistor is not saturat-

ed.

LM3916 Output Circuit

TL/H/7971– 6

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 REplus

the transistors’ collector resistance, is about 50X. 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 mF tantalum or 10 mF aluminum electrolytic capacitor.

MODE PIN USE

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

devices, and controls bar or dot mode operation. The fol-

lowing 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

apin).

Dot Display, Single LM3916 Driver: Leave the Mode Se-

lect 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 Ý9 (pin 11 to VLED).

Mode Pin Functional Description

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

fied block diagram below.

Block Diagram of Mode Pin Function

*High for bar

TL/H/7971– 7

Mode Pin Functional Description (Continued)

DOT OR BAR MODE SELECTION

The voltage at pin 9 is sensed by comparator C1, nominally

referenced to (Vab100 mV). The chip is in bar mode when

pin 9 is above this level; otherwise it’s in dot mode. The

comparator is designed so that pin 9 can be left open circuit

for dot mode.

Taking into account comparator gain and variation in the

100 mV reference level, pin 9 should be no more than 20

mV below Vafor bar mode and more than 200 mV below

Va(or open circuit) for dot mode. In most applications, pin

9 is either open (dot mode) or tied to Va(bar mode). In bar

mode, pin 9 should be connected directly to pin 3. Large

currents drawn from the power supply (LED current, for ex-

ample) should not share this path so that large IR drops are

avoided.

DOT MODE CARRY

In order for display to make sense when multiple drivers are

cascaded in dot mode, special circuitry has been included

to shut off LED Ý10 of the first device when LED Ý1ofthe

second device comes on. The connection for cascading in

dot mode has already been described and is depicted in

Figure 1.

As long as the input signal voltage is below the threshold of

the second driver, LED Ý11 is off. Pin 9 of driver Ý1 thus

sees effectively an open circuit so the chip is in dot mode.

As soon as the input voltage reaches the threshold of LED

Ý11, pin 9 of driver Ý1 is pulled an LED drop (1.5V or

more) below VLED. This condition is sensed by comparator

C2, referenced 600 mV below VLED. This forces the output

of C2 low, which shuts off output transistor Q2, extinguish-

ing LED Ý10.

VLED is sensed via the 20k resistor connected to pin 11.

The very small current (less than 100 mA) that is diverted

from LED Ý9 does not noticeably affect its intensity.

An auxiliary current source at pin 1 keeps at least 100 mA

flowing through LED Ý11 even if the input voltage rises high

enough to extinguish the LED. This ensures that pin 9 of

driver Ý1 is held low enough to force LED Ý10 off when

any

higher LED is illuminated. While 100 mA does not nor-

mally produce significant LED illumination, it may be notice-

able when using high-efficiency LEDs in a dark environment.

If this is bothersome, the simple cure is to shunt LED Ý11

(and LED Ý1) with a 10k resistor. The 1V 1R drop is more

than the 900 mV worst case required to hold off LED Ý10

yet small enough that LED Ý11 does not conduct signifi-

cantly.

In some circuits a number of outputs on the higher device

are not used. Examples include the high resolution VU me-

ter and the expanded range VU meter circuits (see Typical

Applications). To provide the proper carry sense voltage in

dot mode, the LEDs of the higher driver IC are tied to VLED

through two series-connected diodes as shown in

Figure 2.

Shunting the diodes with a 1k resistor provides a path for

driver leakage current.

TL/H/7971– 8

FIGURE 1. Cascading LM3914/15/16 Series in Dot Mode

TL/H/7971– 9

FIGURE 2. Cascading Drivers in Dot Mode with Pin 1 of Driver Ý2 Unused

Mode Pin Functional

Description (Continued)

OTHER DEVICE CHARACTERISTICS

The LM3915 is relatively low-powered itself, and since any

number of LEDs can be powered from about 3V, it is a very

efficient display driver. Typical standby supply current (all

LEDs OFF) is 1.6 mA. However, any reference loading adds

4 times that current drain to the Va(pin 3) supply input. For

example, an LM3915 witha1mAreference pin load (1.3k)

would supply almost 10 mA to every LED while drawing only

10 mA from its Vapin supply. At full-scale, the IC is typically

drawing less than 10% of the current supplied to the dis-

play.

The display driver does not have built-in hysteresis so that

the display does not jump instantly from one LED to the

next. Under rapidly changing signal conditions, this cuts

down high frequency noise and often an annoying flicker.

An ‘‘overlap’’ is built in so that at no time are all segments

completely off the dot mode. Generally one LED fades in

while the other fades out overa1mVrange. The change

may be much more rapid between LED Ý10 of one device

and LED Ý1ofa

second

device cascaded.

Application Hints

The most difficult problem occurs when large LED currents

are being drawn, especially in bar graph mode. These cur-

rents flowing out of the ground pin cause voltage drops in

external wiring, and thus errors and oscillations. Bringing the

return wires from signal sources, reference ground and bot-

tom of the resistor string to a single point very near pin 2 is

the best solution.

Long wires from VLED to LED anode common can cause

oscillations. The usual cure is bypassing the LED anodes

with a 2.2 mF tantalum or 10 mF aluminum electrolytic ca-

pacitor. If the LEd anode line wiring is inaccessible, often a

0.1 mF capacitor from pin 1 to pin 2 will be sufficient.

If there is a large amount of LED overlap in the bar mode,

oscillation or excessive noise is usually the problem. In

cases where proper wiring and bypassing fail to stop oscilla-

tions, Vavoltage at pin 3 is usually below suggested limits.

When several LEDs are lit in dot mode, the problem is usu-

ally an AC component of the input signal which should be

filtered out. Expanded scale meter applications may have

one or both ends of the internal voltage divider terminated

at relatively high value resistors. These high-impedance

ends should be bypassed to pin 2 with 0.1 mF.

Power dissipation, especially in bar mode should be given

consideration. For example, with a 5V supply and all LEDs

programmed to 20 mA the driver will dissipate over 600 mW.

In this case a 7.5Xresistor in series with the LED supply will

cut device heating in half. The negative end of the resistor

should be bypassed with a 2.2 mF solid tantalum or 10 mF

aluminum electrolytic capacitor to pin 2.

TIPS ON RECTIFIER CIRCUITS

The simplest way to display an AC signal using the LM3916

is to apply it right to pin 5 unrectified. Since the LED illumi-

nated represents the instantaneous value of the AC wave-

form, one can readily discern both peak and average values

of audio signals in this manner. The LM3916 will respond to

positive half-cycles only but will not be damaged by signals

up to g35V (or up to g100V if a 39k resistor is in series

with the input). A smear or bar type display results even

though the LM3916 is connected for dot mode. The LEDs

should be run at 20 mA to 30 mA for high enough average

intensity.

True average or peak detection requires rectification. If an

LM3916 is set up with 10V full scale across its voltage divid-

er, the turn-on point for the first LED is only 450 mV. A

simple silicon diode rectifier won’t work well at the low end

due to the 600 mV diode threshold. The half-wave peak

detector in

Figure 3

uses a PNP emitter-follower in front of

the diode. Now, the transistor’s base-emitter voltage can-

cels out the diode offset, within about 100 mV. This ap-

proach is usually satisfactory when a single LM3916 is used

for a 23 dB display.

Display circuits such as the extended range VU meter using

two or more drivers for a dynamic range of 40 dB or greater

require more accurate detection. In the precision half-wave

rectifier of

Figure 4

the effective diode offset is reduced by a

factor equal to the open-loop gain of the op amp. Filter ca-

pacitor C2 charges through R3 and discharges through R2

and R3, so that appropriate selection of these values results

in either a peak or an average detector. The circuit has a

gain equal to R2/R1.

It’s best to capacitively couple the input. Audio sources fre-

quently have a small DC offset that can cause significant

error at the low end of the log display. Op amps that slew

quickly, such as the LF351, LF353 or LF356, are needed to

faithfully respond to sudden transients. It may be necessary

to trim out the op amp DC offset voltage to accurately cover

a 60 dB range. Best results are obtained if the circuit is

adjusted for the correct output when a low-level AC signal

(10 to 20 mV) is applied, rather than adjusting for zero out-

put with zero input.

TL/H/7971– 10

*DC Couple

FIGURE 3. Half-Wave Peak Detector

D1, D2: 1N914 or 1N4148

Average Peak

R2 1k 100k

R3 100k 1k

R1 eR2 for AVe1

R1 eR2/10 for AVe10

C1 e10/R1

TL/H/7971– 11

FIGURE 4. Precision Half-Wave Rectifier

Application Hints (Continued)

For precision full-wave averaging use the circuit in

Figure 5.

Using 1% resistors for R1 through R4, gain for positive and

negative signal differs by only 0.5 dB worst case. Substitut-

ing 5% resistors increases this to 2 dB worst case. (A 2 dB

gain difference means that the display may have a g1dB

error when the input is a nonsymmetrical transient). The

averaging time constant is R5#C2. A simple modification

results in the precision full-wave detector of

Figure 6.

Since

the filter capacitor is not buffered, this circuit can drive only

high impedance loads such as the input of an LM3916.

AUDIO METER STANDARDS

VU Meter

The audio level meter most frequently encountered is the

VU meter. Its characteristics are defined as the ANSI speci-

fication C165. The LM3916’s outputs correspond to the me-

ter indications specified with the omission of the b2VU

indication. The VU scale divisions differ slightly from a linear

scale in order to obtain whole numbers in dB.

Some of the most important specifications for an AC meter

are its dynamic characteristics. These define how the meter

responds to transients and how fast the reading decays.

The VU meter is a relatively slow full-wave averaging type,

specified to reach 99% deflection in 300 ms and overshoot

by 1 to 1.5%. In engineering terms this means a slightly

underdamped second order response with a resonant fre-

quency of 2.1 Hz andaQof0.62.

Figure 7

depicts a simple

rectifier/filter circuit that meets these criteria.

D1, D2: 1N914 or 1N4148

TL/H/7971– 12

FIGURE 5. Precision Full-Wave Average Detector

TL/H/7971– 13

D1, D2, D3, D4: 1N914 OR 1N4148

FIGURE 6. Precision Full-Wave Peak Detector

Attack and decay time to DIN PPM

spec. Response down 1 dB for 10 ms

tone burst. Decays 20 dB in 1.5s.

TL/H/7971– 14

GAIN R5 R6 C2 C3

1 100k 43k 2.0 0.56 mF

10 1M 100k 1.0 0.056 mF

Design Equations

R5 #R6 #C2 #C3 e002e177 sec b2

C2 #1

R3 a1

R4 a1

R5 a1

R6 Je00

Qe21.5 sec b1

R3 e2R4

R1 eR2 mR4

A1, A2: (/2 LF353

D1, D2: 1N914 OR 1N4148

*Reaches 99% level at 300 ms after applied

tone burst and overshoots 1.2%.

FIGURE 7. Full-Wave Average Detector to VU Meter Specifications*

Application Hints (Continued)

Peak Program Meter

The VU meter, originally intended for signals sent via tele-

phone lines, has shortcomings when used in high fidelity

systems. Due to its slow response time, a VU meter will not

accurately display transients that can saturate a magnetic

tape or drive an amplifier into clipping. The fast-attack peak

program meter (PPM) which does not have this problem is

becoming increasingly popular.

While several European organizations have specifications

for peak program meters, the German DIN specification

45406 is becoming a de facto standard. Rather than re-

spond instantaneously to peak, however, PPM specifica-

tions require a finite ‘‘integration time’’ so that only peaks

wide enough to be audible are displayed. DIN 45406 calls

for a response of 1 dB down from steady-state for a 10 ms

tone burst and 4 dB down fora3mstone burst. These

requirements are consistent with the other frequently en-

countered spec of 2 dB down fora5msburst and are met

by an attack time constant of 1.7 ms.

The specified return time of 1.5s to b20 dB requires a 650

ms decay time constant. The full-wave peak detector of

FIGURE 6

satisfies both the attack and decay time criteria.

Cascading The LM3916

The LM3916 by itself covers the 23 dB range of the conven-

tional VU meter. To display signals of 40 dB or 70 dB dy-

namic range, the LM3916 may be cascaded with the 3 dB/

step LM3915s. Alternatively, two LM3916s may be cascad-

ed for increased resolution over a 28 dB range. Refer to the

Extended Range VU Meter and High Resolution VU Meter in

the Typical Applications section for the complete circuits for

both dot and bar mode displays.

To obtain a display that makes sense when an LM3915 and

an LM3916 are cascaded, the b20 dB output from the

LM3916 is dropped. The full-scale display for the LM3915 is

set at 3 dB below the LM3916’s b10 dB output and the rest

of the thresholds continue the 3 dB/step spacing. A simple,

low cost approach is to set the reference voltage of the two

chips 16 dB apart as in

Figure 5

. The LM3915, with pin 8

grounded, runs at 1.25V full-scale. R1 and R2 set the

LM3916’s reference 16 dB higher or 7.89V. Variation in the

two on-chip references and resistor tolerance may cause a

g1 dB error in the b10 dB to b13 dB transition. If this is

objectionable, R2 can be trimmed.

The drawback of the aforementioned approach is that the

threshold of LED Ý1 on the LM3915 is only 56 mV. Since

comparator offset voltage may be as high as 10 mV, large

errors can occur at the first few thresholds. A better ap-

proach, as shown in

Figure 9,

is to keep the reference the

same for both drivers (10V in the example) and

amplify

the

input signal by 16 dB ahead of the LM3915. Alternatively,

VREF1 e1.25V VREF2 j7.89V

7.89V

1.25 e6.31 e16 dB

TL/H/7971– 15

FIGURE 8. Low Cost Circuit for 40 dB Display

R1 aR2

R1 e6.31

e16 dB

TL/H/7971– 16

FIGURE 9. Improved Circuit for 40 dB Display

Application Hints (Continued)

instead of amplifying, input signals of sufficient amplitude

can be fed directly to the LM3916 and

attenuated

by 16 dB

to drive the LM3915.

To extend this approach to get a 70 dB display, another

30 dB of amplification must be placed in the signal path

ahead of the lowest LM3915. Extreme care is required as

the lowest LM3915 displays input signals down to 2 mV!

Several offset nulls may be required. High currents should

not share the same path as the low level signal. Also power

line wiring should be kept away from signal lines.

TIPS ON REFERENCE VOLTAGE AND LED CURRENT

PROGRAMMING

Single Driver

The equations in

Figure 10

illustrate how to choose resistor

values to set reference voltage for the simple case where

no LED intensity adjustment is required. A LED current of

10 mA to 20 mA generally produces adequate illumination.

Having 10V full-scale across the internal voltage divider

gives best accuracy by keeping signal level high relative to

the offset voltage of the internal comparators. However, this

causes 1 mA to flow from pin 7 into the divider which means

that the LED current will be at least 10 mA. R1 will typically

be between 1 kXand5kX. To trim the reference voltage,

vary R2.

The current in

Figure 11

shows how to add a LED intensity

control which can vary LED current from 5 mA to 28 mA.

Choosing VREF e5V lowers the current drawn by the lad-

der, increasing the intensity adjustment range. The refer-

ence adjustment has some effect on LED intensity but the

reverse is not true.

Multiple Drivers

Figure 12

shows how to obtain a common reference trim

and intensity control for two drivers. The two ICs may be

connected in cascade or may be handling separate chan-

nels for stereo. This technique can be extended for larger

numbers of drivers by varying the values of R1, R2 and R3.

Because the LM3915 has a greater ladder resistance, R5

was picked less than R7 in such a way as to provide equal

reference load currents. The ICs’ internal references track

within 100 mV so that worst case error from chip to chip is

only 0.2 dB for VREF e5V.

Adjust R2 to vary VREF

Pick R1 e12.5V

ILED bVREF/1kX

Pick R2 e(VREF b1.25V)

1.25V/R1 a0.08 mA

TL/H/7971– 17

FIGURE 10. Design Equations for Fixed LED Intensity

5mAsI

LED s28 mA @VREF e5V

TL/H/7971– 18

FIGURE 11. Varying LED Intensity

5mAsI

LED s28 mA

@VREF e5V TL/H/7971– 19

FIGURE 12. Independent Adjustment of Reference Voltage and LED Intensity for Multiple Drivers

Application Hints (Continued)

The scheme in

Figure 13

is useful when the reference and

LED intensity must be adjusted independently over a wide

range. The RHI voltage can be adjusted from 1.2V to 10V

with no effect on LED current. Since the internal divider here

does not load down the reference, minimum LED current is

much lower. At the minimum recommended reference load

of 80 mA, LED current is about 0.8 mA. The resistor values

shown give a LED current range from 1.5 mA to 25 mA.

At the low end of the intensity adjustment, the voltage drop

across the 510Xcurrent-sharing resistors is so small that

chip to chip variation in reference voltage may yield a visible

variation in LED intensity. The optional approach shown of

connecting the bottom end of the intensity control pot to a

negative supply overcomes this problem by allowing a larger

voltage drop across the (larger) current-sharing resistors.

Other Applications

For increased resolution, it’s possible to obtain a display

with a smooth transition between LEDs. This is accom-

plished by superimposing an AC waveform on top of the

input level as shown in

Figure 14

. The signal can be a trian-

gle, sawtooth or sine wave from 60 Hz to 1 kHz. The display

can be run in either dot or bar mode.

1.25V sVREF s10V

*Optional circuit for improved intensity 1.5 mA sILED s25 mA

matching at low currents. See text.

TL/H/7971– 20

FIGURE 13. Wide-Range Adjustment of Reference Voltage and LED intensity for Multiple Drivers

TL/H/7971– 21

FIGURE 14. 0V to 10V VU Meter with Smooth Transitions

Typical Applications (Continued)

Extended Range VU Meter (Bar Mode)

TL/H/7971– 22

This application shows that the LED supply re-

quires minimal filtering.

*See Application Hints for optional Peak or Aver-

age Detector.

²Adjust R3 for 3 dB difference between LED Ý11

and LED Ý12

R3 aR4 e0.158 eb

16 dB

Typical Applications (Continued)

Extended Range VU Meter (Dot Mode)

D1, D2: 1N914 or 1N4148

*OPTIONAL SHUNTS 100 mA auxiliary sink cur-

rent away from LED Ý1.

²See Application Hints for optional peak or aver-

TL/H/7971– 23

age detector.

Typical Applications (Continued)

Driving Vacuum Fluorescent Display

R7 thru R15: 10kg10%

D1, D2: 1N914 or 1N4148

*Half-wave peak detector.

See Application Hints.

TL/H/7971– 24

Indicator and Alarm, Full-Scale Changes Display From Dot to Bar

TL/H/7971– 25

*The input to the Dot-Bar switch may

be taken from cathodes of other

LEDs. Display will change to bar as

soon as the LED so selected begins

to light.

**Optional. Shunts 100 mA auxiliary

sink current away from LED Ý1.

Typical Applications (Continued)

High Resolution VU Meter (Bar Mode)

TL/H/7971– 26

*See Application Hints for optional peak or aver-

age detector.

R1 aR2 j0.562 eb

5dB

or R1 j0.788 #R2

Typical Applications (Continued)

High Resolution VU Meter (Dot Mode)

TL/H/7971– 27

*Optional shunts 100 mA auxiliary sink current

away from LED Ý1.

²See Application Hints for optional peak or aver-

age detector.

R1 aR2 j0.562 eb

5dB

or R1 j0.788 #R2

Typical Applications (Continued)

Displaying Additional Levels

R2 aR3 j0.794 eb

2dB

R2 aR3

R1 aR2 aR3 j0.562 eb

5dB

or R2 e0.259 #R3

and R1 e0.979 #R3

TL/H/7971– 28

Operating with a High Voltage Supply (Dot Mode Only)

The LED currents are approximately 10 mA, and

LM3916 outputs operate in saturation for mini-

mum dissipation.

*This point is partially regulated and decreases in

voltage with temperature. Voltage requirements

of the LM3916 also decrease with temperature.

TL/H/7971– 29

Typical Applications (Continued)

Low Current Bar Mode Display

Supply current drain is only 20 mA

with ten LEDs illuminated @16 mA. TL/H/7971– 30

Driving Liquid Crystal Display

TL/H/7971– 31

Bar Display with Alarm Flasher

Full-scale causes the full bar display to

flash. If the junction of R1 and C1 is con-

nected to a different LED cathode, the dis-

play will flash when that LED lights, and at TL/H/7971– 32

any higher input signal.

Connection Diagram

Dual-In-Line Package

TL/H/7971– 33

Top View

Order Number LM3916N

See NS Package Number N18A

Definition of Terms

Absolute Accuracy: The difference between the observed

threshold voltage and the ideal threshold voltage for each

comparator. Specified and tested with 10V across the inter-

nal voltage divider so that resistor ratio matching error pre-

dominates over comparator offset voltage.

Adjust Pin Current: Current flowing out of the reference

amplifier pin when the reference amplifier is in the linear

region.

Comparator Gain: The ratio of the change in output current

(ILED) to the change in input voltage (VIN) required to pro-

duce it for a comparator in the linear region.

Dropout Voltage: The voltage measured at the current

source outputs required to make the output current fall by

10%.

Input Bias Current: Current flowing out of the signal input

when the input buffer is in the linear region.

LED Current Regulation: The change in output current

over the specified range of LED supply voltage (VLED)as

measured at the current source outputs. As the forward volt-

age of an LED does not change significantly with a small

change in forward current, this is equivalent to changing the

voltage at the LED anodes by the same amount.

Line Regulation: The average change in reference output

voltage (VREF) over the specified range of supply voltage

(Va).

Load Regulation: The change in reference output voltage

over the specified range of load current (IL(REF)).

Offset Voltage: The differential input voltage which must

be applied to each comparator to bias the output in the

linear region. Most significant error when the voltage across

the internal voltage divider is small. Specified and tested

with pin 6 voltage (VRHI) equal to pin 4 voltage (VRLO).

Relative Accuracy: The difference between any two adja-

cent threshold points. Specified and tested with 10V across

the internal voltage divider so that resistor ratio matching

error predominates over comparator offset voltage.

LM3916 Dot/Bar Display Driver

Physical Dimensions inches (millimeters)

Molded Dual-In-Line Package (N)

Order Number LM3916N

NS Package Number N18A

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