LM3914

Manufacturer Part NumberLM3914
ManufacturerTexas Instruments
LM3914 datasheet
 


Specifications of LM3914

Output Channels10Led Voltage(max)(v)17
Pin/package18PDIP, 20PLCCRegulated Outputs(#)10
Iout(max)(a)0.03Led ConfigurationParallel
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Functional Description
The simplifed LM3914 block diagram is 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
125mV 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 1.5V below V
and no less than V
meter display is desired, the total divider voltage can be as
little as 200mV. Expanded-scale meter displays are more
accurate and the segments light uniformly only if bar mode is
used. At 50mV or more per step, dot mode is usable.
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
then flows through the output set resistor
1
R2 giving an output voltage of:
Since the 120µA current (max) from the adjust terminal
represents an error term, the reference was designed to
minimize changes of this current with V
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 displays
or ways of indicating input overvoltages, alarms, etc.
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MODE PIN USE
Pin 9, the Mode Select input controls chaining of multiple
LM3914s, 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 applica-
tions.
Bar Graph Display: Wire Mode Select (pin 9) directly to pin
+
3 (V
pin).
Dot Display, Single LM3914 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
. If an expanded scale
comparison points) to pin 1 of the next higher LM3914 driver.
Continue connecting pin 9 of lower input drivers to pin 1 of
higher input drivers for 30, 40, or more LED displays. The
last LM3914 driver in the chain will have pin 9 wired to pin 11.
All previous drivers should have a 20k resistor in parallel with
LED No. 9 (pin 11 to V
Mode Pin Functional Description
This pin actually performs two functions. Refer to the simpli-
fied block diagram below.
Block Diagram of Mode Pin Description
00797004
*High for bar
+
and load changes.
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
100mV reference level, pin 9 should be no more than 20mV
+
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.
8
).
LED
+
− 100mV). The chip is in bar mode when
for bar mode and more than 200mV below V
+
(bar mode). In bar mode,
00797005
+
(or