LM3915MWC National Semiconductor Corporation, LM3915MWC Datasheet - Page 9

• Manufacturer Part Number: LM3915MWC
• Description: Dot/bar Display Driver
• Manufacturer: National Semiconductor Corporation

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 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 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
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
divider, 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 cancels
out the diode offset, within about 100 mV. This approach is
usually satisfactory when a single LM3915 is used for a 30
dB display.
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
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
frequently 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 mV to 20 mV) is applied, rather than adjusting for zero
output 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
±
35V (or up to
±
100V if a 39k resistor is in series with
(Continued)
9
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
R1 = R2 for A
R1 = R2/R10 for A
C1 = 10/R1
DC Couple
R2
R3
Average
FIGURE 2. Precision Half-Wave Rectifier
100k
FIGURE 1. Half-Wave Peak Detector
1k
V
= 1
V
= 10
Peak
100k
1k
DS005104-10
DS005104-9
www.national.com
±
1 dB