The LM3915 is a monolithic integrated circuit that senses
analog voltage levels and drives ten LEDs, LCDs or vacuum
fluorescent displays, providing a logarithmic 3 dB/step analog display.Onepinchanges the display from a bar graph to
a moving dot display. LED current drive is regulated and
programmable, eliminating the need for current limiting 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
accurate 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
parators referenced to the precision divider. Accuracy is
typically better than 1 dB.
The LM3915’s 3 dB/step display is suited for signals with
wide dynamic range, such as audio level, power, light intensity or vibration. Audio applications include average or peak
level indicators, power meters and RF signal strength
meters. Replacing conventional meters with an LED bar
graph results in a faster responding, more rugged display
with high visibility that retains the easeofinterpretation of an
analog display.
The LM3915 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.
±
35V. The input buffer drives 10 individual com-
The LM3915 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 with a range of 60 or 90 dB.
LM3915s can also be cascaded with LM3914s for a linear/
log display or with LM3916s for an extended-range VU
meter.
Features
n 3 dB/step, 30 dB range
n Drives LEDs, LCDs, or vacuum fluorescents
n Bar or dot display mode externally selectable by user
n Expandable to displays of 90 dB
n Internal voltage reference from 1.2V to 12V
n Operates with single supply of 3V to 25V
n Inputs operate down to ground
n Output current programmable from 1 mA to 30 mA
n Input withstands
n Outputs are current regulated, open collectors
n Directly drives TTL or CMOS
n The internal 10-step divider is floating and can be
referenced to a wide range of voltages
The LM3915 is rated for operation from 0˚C to +70˚C. The
LM3915N-1 is available in an 18-lead molded DIP package.
Notes: Capacitor C1 is required if leads to the LED supply are 6" or longer.
Circuit as shown is wired for dot mode. For bar mode, connect pin 9 to pin 3. V
dissipation.
0V to 10V Log Display
must be kept below 7V or dropping resistor should be used to limit IC power
LED
DS005104-1
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LM3915
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Power Dissipation (Note 6)
Molded DIP(N)1365 mW
Supply Voltage25V
Voltage on Output Drivers25V
Input Signal Overvoltage (Note 4)
±
Divider Voltage−100 mV to V
Reference Load Current10 mA
Storage Temperature Range−55˚C to +150˚C
Lead Temperature
(Soldering, 10 sec.)260˚C
Electrical Characteristics (Notes 2, 4)
ParameterConditions (Note 2)MinTypMaxUnits
COMPARATOR
Offset Voltage, Buffer and First
Comparator
Offset Voltage, Buffer and Any Other
Comparator
Gain (∆I
/∆VIN)I
LED
Input Bias Current (at Pin 5)0V ≤ V
Input Signal OvervoltageNo Change in Display−3535V
VOLTAGE-DIVIDER
Divider ResistanceTotal, Pin 6 to 4162836kΩ
Relative Accuracy (Input Change
Between Any Two Threshold Points)
Absolute Accuracy at Each Threshold
Point
VOLTAGE REFERENCE
Output Voltage0.1 mA ≤ I
Line Regulation3V ≤ V
Load Regulation0.1 mA ≤ I
Output Voltage Change with
Temperature
Adjust Pin Current75120µA
OUTPUT DRIVERS
LED CurrentV
LED Current Difference (Between
Largest and Smallest LED Currents)
LED Current Regulation2V ≤ V
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
LM3915
functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which
guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit
is given, however, the typical value is a good indication of device performance.
Note 2: Unless otherwise stated, all specifications apply with the following conditions:
3V
≤ V+≤ 20 V
DC
3V
≤ V
LED
RHI
≤ V
≤ 12 V
DC
−0.015V ≤ V
Note 3: Accuracyis measured referred to 0 dB = + 10.000 V
comparator offset voltage may add significant error. See table for threshold voltages.
Note 4: Pin 5 input current must be limited to
Note 5: Bar mode results when pin 9 is within 20 mV of V
disabled if pin 9 is pulled 0.9V or more below V
Note 6: Themaximum junction temperature of the LM3915 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).
DC
+
−0.015V ≤ V
V
REF,VRHI,VRLO
0V ≤ VIN≤ V+− 1.5V
DC
≤ 12 V
RLO
≤ (V+− 1.5V)For higher power dissipations, pulse testing is used.
±
3 mA. The addition of a 39k resistor in series with pin 5 allows±100V signals without damage.
.
LED
TA= 25˚C, I
DC
at pin 5, with + 10.000 VDCat pin 6, and 0.000 VDCat pin 4. At lower full scale voltages, buffer and
DC
+
. Dot mode results when pin 9 is pulled at least 200 mV below V+. LED#10 (pin 10 output current) is
The simplified LM3915 block diagram is included to give the
general idea of the circuit’s operation. A high input impedance buffer operates with signals from ground to 12V, and is
protected against reverse and overvoltage signals. The signal 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. In this case, for each
3 dB that the input signal increases, a comparator will switch
on another indicating LED. This resistor divider can be connected between any 2 voltages, providing that they are at
least 1.5V below 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
R2 giving an output voltage of:
Since the 120 µA current (max) from the adjust terminal
represents an error term, the reference was designed to
minimize changes of this current with V
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
reference voltage pin (pin 7) determines LED current. Approximately 10 times this current will be drawn through each
lighted LED, and this current will be relatively constant despite 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 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 LM3915 outputs are current-limited NPN transistors as
shown below. An internal feedback loop regulates the transistor 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 saturated.
+
and no lower than V−.
then flows through the output set resistor
1
DS005104-5
+
and load changes.
LM3915 Output Circuit
DS005104-6
Outputs may be run in saturation with no adverse effects,
making it possible to directly drive logic. The effective saturation resistance of the output transistors, equal to R
plus
E
the transistors’ collector resistance, is about 50Ω. 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 µF tantalum or 10 µF aluminum electrolytic capacitor.
MODE PIN USE
Pin 9, the Mode Select input, permits chaining of multiple
LM3915s, 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)
+
3(V
pin).
directly
to pin
Dot Display, Single LM3915 Driver: Leave the Mode Select
pin open circuit.
Dot Display, 20 or More LEDs: Connect pin 9 of the
first
driver in the series (i.e., the one with the lowest input voltage
comparison points) to pin 1 of the next higher LM3915 driver.
Continue connecting pin 9 of lower input drivers to pin 1 of
higher input drivers for 30 or more LED displays. The last
LM3915 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 simplified block diagram below.
Block Diagram of Mode Pin Function
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*
High for bar
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Mode Pin Functional Description
(Continued)
LM3915
DOT OR BAR MODE SELECTION
The voltage at pin 9 is sensed by comparator C1, nominally
referenced to (V
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 V
+
open circuit) for dot mode. In most applications, pin 9 is
either open (dot mode) or tied to V
pin 9 should be connected directly to pin 3. Large currents
drawn from the power supply (LED current, for example)
should not share this path so that large IR drops are avoided.
DOT MODE CARRY
In order for the display to make sense when multiple
LM3915s are cascaded in dot mode, special circuitry has
been included to shut off LED
#
LED
1 of the second device comes on. The connection for
cascading in dot mode has already been described and is
depicted below.
As long as the input signal voltage is below the threshold of
the second LM3915, LED
sees effectively an open circuit so the chip is in dot mode.As
soon as the input voltage reaches the threshold of LED
pin 9 of LM3915
below V
referenced 600 mV below V
low, which shuts off output transistor Q2, extinguishing LED
#
10.
V
is sensed via the 20k resistor connected to pin 11. The
LED
very small current (less than 100 µA) that is diverted from
#
LED
9 does not noticeably affect its intensity.
An auxiliary current source at pin 1 keeps at least 100 µA
flowing through LED
enough to extinguish the LED. This ensures that pin 9 of
LM3915
any
higher LED is illuminated. While 100 µA does not normally produce significant LED illumination, it may be noticeable when using high-efficiency LEDs in a dark environment.
If this is bothersome, the simple cure is to shunt LED
with a 10k resistor. The 1V IR drop is more than the 900 mV
worst case required to hold off LED
that LED
+
− 100 mV). The chip is in bar mode when
for bar mode and more than 200 mV below V+(or
+
(bar mode). In bar mode,
#
10 of the first device when
#
11is off. Pin 9 of LM3915#1 thus
#
1 is pulled an LED drop (1.5V or more)
. This condition is sensed by comparator C2,
LED
#
11 even if the input voltage rises high
#
1 is held low enough to force LED#10 off when
#
11 does not conduct significantly.
. This forces the output of C2
LED
#
10 yet small enough
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 LM3916 witha1mAreference 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 “overlap” is built in so that at no time are all segments completely
off in the dot mode. Generally 1 LED fades in while the other
fades out over a mV or more of range. The change may be
much more rapid between LED
#
1ofa
second
device “chained” to the first.
#
10 of one device and LED
Application Hints
The most difficult problem occurs when large LED currents
are being drawn, especially in bar graph mode. These currents 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 bottom of the resistor string to a single point very near pin 2 is
#
11,
#
11
the best solution.
Long wires from V
to LED anode common can cause
LED
oscillations. Depending on the severity of the problem
0.05 µF to 2.2 µF decoupling capacitors from LED anode
common to pin 2 will damp the circuit. If LED anode line
wiring is inaccessible, often similar decoupling from pin 1 to
pin 2 will be sufficient.
If LED turn ON seems slow (bar mode) or several LEDs light
(dot mode), oscillation or excessive noise is usually the
problem. In cases where proper wiring and bypassing fail to
stop oscillations, V
+
voltage at pin 3 is usually below suggested limits. 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 at least a 0.001 µF capacitor, or up to 0.1 µF in noisy environments.
Cascading LM3915s in Dot Mode
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DS005104-8
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