LM3915N National Semiconductor, LM3915N Datasheet - Page 8

• Manufacturer Part Number: LM3915N
• Description: IC DRIVER DOT BAR DISPLAY 18-DIP
• Manufacturer: National Semiconductor

Specifications of LM3915N

• Display Type: LED, LCD, Vacuum Fluorescent
• Configuration: Dot/Bar Display
• Digits Or Characters: 10 Steps
• Current - Supply: 6.1mA
• Voltage - Supply: 3 V ~ 25 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Through Hole
• Package / Case: 18-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Interface: -
• Other names: *LM3915N; LM3915

Application Hints
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
cut device heating in half The negative end of the resistor
should be bypassed with a 2 2 F solid tantalum capacitor
to pin 2
TIPS ON RECTIFIER CIRCUITS
The simplest way to display an AC signal using the LM3915
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 LM3915 will respond to
positive half-cycles only but will not be damaged by signals
up to
with the input) It’s recommended to use dot mode and to
run the LEDs at 30 mA for high enough average intensity
True average or peak detection requires rectification If an
LM3915 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 1 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 LM3915 is used
for a 30 dB display
g
35V (or up to
FIGURE 1 Half-Wave Peak Detector
resistor in series with the LED supply will
g
100V if a 39k resistor is in series
(Continued)
FIGURE 3 Precision Full-Wave Average Detector
DC Couple
TL H 5104– 9
8
Display circuits using two or more LM3915s for a dynamic
range of 60 dB or greater require more accurate detection
In the precision half-wave rectifier of Figure 2 the effective
diode offset is reduced by a factor equal to the open-loop
gain of the op amp Filter capacitor C2 charges through R3
and discharges through R2 and R3 so that appropriate se-
lection 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
For precision full-wave averaging use the circuit in Figure 3
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
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 4 Since
the filter capacitor is not buffered this circuit can drive only
high impedance loads such as the input of an LM3915
D1 D2 1N914 or 1N4148
FIGURE 2 Precision Half-Wave Rectifier
TL H 5104 – 11
D1 D2 1N914 or 1N4148
R1
R1
C1
R2
R3
e
e
e
10 R1
R2 for A
R2 R10 for A
Average
100k
TL H 5104 – 10
1k
V
g
e
1 dB
Peak
100k
1
V
1k
e
10