LM3914DWF National Semiconductor Corporation, LM3914DWF Datasheet - Page 7

• Manufacturer Part Number: LM3914DWF
• Description: LM3914 - Dot/bar Display Driver, Package: Mdip, Pin Nb=18
• Manufacturer: National Semiconductor Corporation

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 bi-
ased 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
125 mV 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
meter display is desired, the total divider voltage can be as
little as 200 mV. Expanded-scale meter displays are more
accurate and the segments light uniformly only if bar mode is
used. At 50 mV 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
R2 giving an output voltage of:
Since the 120 µA current (max) from the adjust terminal rep-
resents an error term, the reference was designed to mini-
mize 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 ref-
erence voltage pin (pin 7) determines LED current. Approxi-
mately 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 ex-
ternal 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.
+
1
and no less than V
then flows through the output set resistor
+
−
and load changes.
. If an expanded scale
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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 fol-
lowing 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
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
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.
*
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 com-
parator 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 ei-
ther 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
LM3914s are cascaded in dot mode, special circuitry has
been included to shut off LED No. 10 of the first device when
High for bar
+
pin).
+
Block Diagram of Mode Pin Description
for bar mode and more than 200 mV below V
+
− 100 mV). The chip is in bar mode when
LED
).
+
(bar mode). In bar mode,
www.national.com
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(or