LM3915N National Semiconductor, LM3915N Datasheet - Page 7

• 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

any higher LED is illuminated While 100 A does not nor-
Mode Pin Functional Description
(Continued)
DOT OR BAR MODE SELECTION
The voltage at pin 9 is sensed by comparator C1 nominally
referenced to (V
when 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
V
9 is either open (dot mode) or tied to V
mode pin 9 should be connected directly to pin 3 Large
currents drawn from the power supply (LED current for ex-
ample) 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
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
thus sees effectively an open circuit so the chip is in dot
mode As soon as the input voltage reaches the threshold
of LED
(1 5V or more) below V
comparator C2 referenced 600 mV below V
the output of C2 low which shuts off output transistor Q2
extinguishing LED
V
The very small current (less than 100
from LED
An auxiliary current source at pin 1 keeps at least 100
flowing through LED
enough to extinguish the LED This ensures that pin 9 of
LM3915
mally produce significant LED illumination it may be notice-
able 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
LED
a
(or open circuit) for dot mode In most applications pin
is sensed via the 20k resistor connected to pin 11
1 of the second device comes on The connection for
11 pin 9 of LM3915
1 is held low enough to force LED
11 does not conduct significantly
a
9 does not noticeably affect its intensity
for bar mode and more than 200 mV below
a
b
10
11 even if the input voltage rises high
100 mV) The chip is in bar mode
LED
11 is off Pin 9 of LM3915
This condition is sensed by
10 of the first device when
1 is pulled an LED drop
a
10 yet small enough
A) that is diverted
(bar mode) In bar
LED
Cascading LM3915s in Dot Mode
10 off when
This forces
11
A
1
7
OTHER DEVICE CHARACTERISTICS
The LM3916 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
example an LM3916 with a 1 mA reference pin load (1 3k)
would supply almost 10 mA to every LED while drawing only
10 mA from its V
drawing less than 10% of the current supplied to the dis-
play
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
device and LED
first
Application Hints
The most difficult problem occurs when large LED currents
are being drawn especially in bar graph mode These cur-
rents 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 bot-
tom of the resistor string to a single point very near pin 2 is
the best solution
Long wires from V
oscillations Depending on the severity of the problem
0 05
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
gested 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 F
capacitor or up to 0 1 F in noisy environments
F to 2 2
a
F decoupling capacitors from LED anode
pin supply At full-scale the IC is typically
LED
a
1 of a second device ‘‘chained’’ to the
voltage at pin 3 is usually below sug-
to LED anode common can cause
a
(pin 3) supply input For
TL H 5104– 8
10 of one