LM3916 datasheet(9/27 Pages) NSC

9 / 27 page

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 V

+ (pin 3) supply input. For

example, an LM3915 witha1mA reference pin load (1.3k)

would supply almost 10 mA to every LED while drawing only

10 mA from its V

+ pin supply. At full-scale, the IC is typically

drawing less than 10% of the current supplied to the display.

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 “over-

lap” 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 overa1mV range. The change may be much

more rapid between LED #10 of one device and LED #1ofa

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 ex-

ternal 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 V

LED to LED anode common can cause os-

cillations. The usual cure is bypassing the LED anodes with

a 2.2 µF tantalum or 10 µF aluminum electrolytic capacitor. If

the LED anode line wiring is inaccessible, often a 0.1 µF ca-

pacitor from pin 1 to pin 2 will be sufficient.

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

cillation or excessive noise is usually the problem. In cases

where proper wiring and bypassing fail to stop oscillations,

+ voltage at pin 3 is usually below suggested limits. When

several LEDs are lit in dot mode, the problem is usually 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 rela-

tively high value resistors. These high-impedance ends

should be bypassed to pin 2 with 0.1 µF.

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 7.5

Ω resistor 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 µF solid tantalum or 10 µF

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 ±35V (or up to ±100V 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 di-

vider, 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 de-

tector in

Figure 3 uses a PNP emitter-follower in front of the

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

the diode offset, within about 100 mV. This approach is usu-

ally 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 er-

ror 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 ad-

justed for the correct output when a low-level AC signal (10

to 20 mV) is applied, rather than adjusting for zero output

with zero input.

DS007971-10

*DC Couple

FIGURE 3. Half-Wave Peak Detector

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