LM3914DWF National Semiconductor Corporation, LM3914DWF Datasheet - Page 8

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

www.national.com
Mode Pin Functional Description
LED No. 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 LM3914, LED No. 11 is off. Pin 9 of LM3914
No. 1 thus sees effectively an open circuit so the chip is in
dot mode. As soon as the input voltage reaches the thresh-
old of LED No. 11, pin 9 of LM3914 No. 1 is pulled an LED
drop (1.5V or more) below V
comparator C2, referenced 600 mV below V
the output of C2 low, which shuts off output transistor Q2, ex-
tinguishing LED No. 10.
V
very small current (less than 100 µA) that is diverted from
LED No. 9 does not noticeably affect its intensity.
An auxiliary current source at pin 1 keeps at least 100 µA
flowing through LED No. 11 even if the input voltage rises
high enough to extinguish the LED. This ensures that pin 9 of
LM3914 No. 1 is held low enough to force LED No. 10 off
when any higher LED is illuminated. While 100 µA does not
normally produce significant LED illumination, it may be no-
ticeable when using high-efficiency LEDs in a dark environ-
ment. If this is bothersome, the simple cure is to shunt LED
No. 11 with a 10k resistor. The 1V IR drop is more than the
900 mV worst case required to hold off LED No. 10 yet small
enough that LED No. 11 does not conduct significantly.
OTHER DEVICE CHARACTERISTICS
The LM3914 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
(Continued)
LED
is sensed via the 20k resistor connected to pin 11. The
LED
. This condition is sensed by
LED
Cascading LM3914s in Dot Mode
. This forces
8
LEDs OFF) is 1.6 mA (2.5 mA max). However, any reference
loading adds 4 times that current drain to the V
ply input. For example, an LM3914 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
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 “over-
lap” is built in so that at no time between segments are all
LEDs completely OFF in the dot mode. Generally 1 LED
fades in while the other fades out over a mV or more of
range (Note 3). The change may be much more rapid be-
tween LED No. 10 of one device and LED No. 1 of a second
device “chained” to the first.
The LM3914 features individually current regulated LED
driver transistors. Further internal circuitry detects when any
driver transistor goes into saturation, and prevents other cir-
cuitry from drawing excess current. This results in the ability
of the LM3914 to drive and regulate LEDs powered from a
pulsating DC power source, i.e., largely unfiltered. (Due to
possible oscillations at low voltages a nominal bypass ca-
pacitor consisting of a 2.2 µF solid tantalum connected from
the pulsating LED supply to pin 2 of the LM3914 is recom-
mended.) This ability to operate with low or fluctuating volt-
ages also allows the display driver to interface with logic cir-
cuitry, opto-coupled solid-state relays, and low-current
incandescent lamps.
+
pin supply. At full-scale, the
+
DS007970-6
(pin 3) sup-